Mod #67: Holding Tank Monitor System

Mod #67: Holding Tank Monitor System

Submitted on: 01/12/09

     Category: featured, plumbing
Mod Rating: 12345

(276 ratings)

Loading ... Loading ...
Viewed: 141267 times
Share your experiences and connect with others who’ve actually done what’s described in this mod. If that’s YOU, click the “I Did This Mod!” button. See the FAQ for more information on this very cool feature!
Congratulations! You've “Done the Mod”. You should see your gravatar displayed in all it's glory over in the sidebar. Don't have a gravatar? Visit and sign up for one. It's free and once you do, your new gravatar will appear in the sidebar and in the comments you make on any mod. See the FAQ for more information.

Mod Description:

Why is it that the simplest solutions often go undiscovered? Take for example holding tank monitors. They are notoriously inaccurate and mostly useless at giving the discriminating RVer the comforting feeling of knowing just how much holding tank capacity or fresh water he has left. Now you might argue that most RV manufacturers try to control cost by using cheap parts, but a tank monitor is a no-brainer, something that for a few extra bucks can make most RVers in to happy campers.

Mod Difficulty:

So what makes these so called tank monitors so inaccurate? Most RVs come with a monitoring system that can be best described as “idiot lights”. You press a button on the monitor and a number of little lights come on indicating the levels of liquid in your tanks, 2 lights equals 1/3 full, 3 lights equals 2/3 full, 4 equals full. 1 light equals empty. Or does it? This is the frustrating part. As simple as these monitors are, they aren’t telling the full picture.

Most tank monitoring systems today consist of a bank of sensors mounted through the side of a holding tank at the Empty, 1/3, 2/3, and Full positions. Whenever the fluid inside the tank contacts these sensors a light will light up indicating just how full that tank is. These sensors are the same as used on fresh water, gray water, and black water holding tanks. The problem is that these they are susceptible to fouling. Various tricks range from dumping water softener or other chemicals into the tanks to dumping bags of crushed ice into the tank and then driving around to knock the crud off of the sensors.

Even when the sensors are clean, they still don’t give really accurate readings. If you have a gray water tank that is filled to around 60%, your monitor lights will indicate that it is only 1/3 full because the water level has not yet reached the 2/3 sensor. Conversely, your fresh water may be between 34% and 65% full and it will indicate 1/3 on your monitor display. As soon as you run a quart of water through the faucet, that light may read empty, even if you were at the 34% level. Or it will still read 1/3 if you were at a higher level. You just don’t know exactly what you have.

Fortunately there is a relatively simple solution. SeeLevel makes a variety of monitoring systems suitable for almost any RV, from tent trailer to luxury diesel pusher. These monitoring systems work completely different from the standard fare. Rather than using through-the-tank sensors, the SeeLevel sensors are mounted on a flexible “electronic strip” that adheres to the outside of the holding tank. They can never be fouled or contaminated as they never see the inside of the tank. Well it sees it but only though some magic radar system.

From the outside of the tank, the sensors determine the tanks level by the density difference of the liquid as measured between all the sensors on the strip. So it is highly accurate and virtually foolproof. And instead of LED lights, the monitor displays the tanks levels using a percentage, which is arguably easier to interpret than some fraction.

Installation is much easier than understanding how the sensors work. There are basically two ways to install the SeeLevel monitoring system. The first method is to use the existing tank sensor wires by relocating the wires from the old sensors to the new sensor strips, and then replacing the existing monitoring panel with the new SeeLevel display. The second method is to start from scratch and run new wires. If you opt for the second method, consider where you will mount the display and how you will route the new wiring.

The easiest way to do this mod is to reuse the existing sensor wiring. This is what most folks choose as it gets you quickly to holding tank level measuring capacity nirvana. Start by locating where you want to mount the display. Depending on where the existing level monitor is, you may be able to directly replace it. But some installations require some trimming to accommodate the SeeLevel display, as the display unit has to be recessed in to the hole to flush mount it. Follow the recommendations in the installation manual to determine the size hole required and cut to fit.

Remove the current monitor and detach wiring, paying careful attention to which wires came from where. Most wiring is color coded but you never know. You can mark them with a piece of masking tape wrapped around the wire with the location written on the tape. Keep the wiring hanging out of the hole and carefully cut the hole to the appropriate size. The SeeLevel comes with a pigtail connector that makes it very easy to connect the factory sensor wiring to the display. Connect the factory wiring to pigtail first following the wiring table in the directions. Then connect the pigtail to the display. Slip the display in to the hole and secure.

The next step is to mount the sensors and connect them to the factory level sensor wiring. Find a flat vertical area on the side of your holding tanks to fit the sensor strips. This could be a bit of a challenge depending on where your holding tanks are located. Motorhomes tend to provide the easiest access to the holding tanks as they are usually just behind a bay door next to the holding tank controls and valves. For travel trailers and fifth wheels, you may have to remove the sheeting covering your holding tanks to gain access. Some trailers have them, some don’t. Just an FYI.

Be sure the area you selected for the sensor strip on the side of the tank is large enough so the whole width of the sender is in contact with the side of the tank, all the way from the top to the bottom of the tank. Make sure that any metal is at least an inch away from the sender. Clean the area well so that there is no dust, grease, oil, water, etc., that would prevent the adhesive on the sender from sticking. Now here is the cool part. If the sensor strip is too long for the height of your tank, you can cut them. What? Yup. The strips are designed to be cut to length to a minimum height of 6″. The sender ends should be 1/4″ to 3/4″ away from the top and bottom of the tank, to allow for the thickness of the tank top and bottom and any bows in them.

Now peel the adhesive backing from the sensor strip, square up the strip with the top and bottom of the tank, and stick on. As with the display wiring, the sensor wiring is really easy. Simply disconnect the sensor wires from the factory sensors and connect them to the corresponding sensor strip wiring, following the instructions. Splicing connectors are provided in the kit to make the job simple. Repeat this for the other sensor strips. Now all that’s left is to calibrate the display and your done.

Einstein once said that things should be made a simple as possible but not any simpler. Knowing your tanks levels to the highest accuracy possible is now simple. Does that make you as smart as Einstein? You be the judge.

TipThere are many models of the SeeLevel monitoring system. Only the most basic model installation was described in this mod. Other models add battery voltage, indoor temperature, water pump switches, water heater pilot light indicators, etc., and various combinations of these features. And they are still just as simple to install.
TipYou can install more than one display unit using the same sensors. A convenient location is near the holding tank valves. If you have an enclosed water bay or a way to waterproof the display panel, adding a second display can help you know when your tanks are empty at the dump station without leaving the comfort of the sewer drain hole.

ModMyRV recommends these parts for this mod (may differ from what is described in the mod):
Tank Monitor System - Monitors up to four tanks from one convenient location

General Reference

Share the mod love:
  • Facebook
  • TwitThis
  • Digg
  • StumbleUpon
  • MySpace
  • Google
  • LinkedIn
  • Pownce
  • Reddit
  • Technorati
  • Yahoo! Buzz
  • E-mail this story to a friend!
Print This Mod Print This Mod

27 Comments For This Mod

Sort by:

  1. Bob Vaughn

    I just found out that on the pickup forum on the Trailer Life Forums web site there is a posting of this unit catching fire in a pickup camper…Lord knows what would have happened had the owner not been there and smelled it burning….Take a look at the photos and read the responses from the company…Scared me out of doing this mod….

  2. ModMyRV

    Here is the link to that thread:

    It’s interesting to note that it appears as though the unit fried because the current passing through one of the switches exceeded the capacity of the switches current rating. And although there was no documentation specifying maximum current for the switch (shame on the company!), it doesn’t look like it’s a defective unit.

    But that is just based on what I read on the thread. Let’s watch this one closely to see what the outcome ends up being. Thanks for letting us all know Bob.

  3. Bob Vaughn

    Just like in football “upon further review” I wonder if it would not be safer to either not hook up the pump switch or buy the model that is just the sensors…..Also what other make works the same as the SeeLevel? I really like the concept as my sensors will show empty when full and full when empty and any where in between……

  4. ModMyRV

    So following the football metaphor, it’s a “safety”. In this situation, if I had one installed, I would disconnect the wiring to the pump switch and reconnect it to the factory pump switch until I heard from the factory about what is happening. I might even revert back to the factory gauges just in case.

    Sensa Tank makes a system similar to the See Level. It’s not as sophisticated but might be a good alternative given the issue. Here is a link:

  5. Bob Vaughn

    I think when I get my stimulus and bail out money I will do this mod…..My TT belly is enclosed so I would have to remove that plus I would probably have to raise my TT so I could work underneath it…I don’t work very well laying on my back…..

  6. ModMyRV

    Now that’s the spirit Bob. By doing this mod though, you do realize that you will be lowering the number of sales for the tire companies by never having a blowout again, right? Goodyear is already laying off thousands…;)

  7. jmt

    I use my tank monitors all the time. Love them. I seem to always be filling up my black water after 2 days. Not sure if I have a flow issue or something else. I need to investigate.

  8. Tim4sales

    Here is a simple system that will tell you when tanks are full or empty. Looks like they only sense single points, but the technology looks similar.

  9. Craig (DaDoc)

    My question (maybe silly, but here goes) would be: My greay and black water tanks are black fiberglass while my fresh water is opaque neoprene. Can these sensors ‘look’ through the black wall of the holding tanks to “see” the levels?

  10. ModMyRV

    @Craig (DaDoc) - My understanding of how these sensors read tank levels is by using a radio signal directed through the tank wall. There is no “visual” or optical level determination. So these sensors should work for most any RV holding tank, including fiberglass.

  11. Mike Wagner

    Is there a kit that will allow this system to be connected to a Silver Leaf monitor?

  12. Bluebird Bob

    I use the MicroPulse system. Just put one hole in tank and connect the wires. It works off pressure of the stuff in the tank. Calibrate it empty and full and it does the rest. Had mine for 4 years now and is very accurate.
    You can Google MicroPulse and find them.

  13. Rick

    You should always use an isolation relay for high current loads - Basic 12V wiring…

  14. Gary Weber

    This is a great mod just add a Fuse or Breaker to be safe!

  15. Chris Ferro

    You may also want to check out the monitors from Electrosense. They are “in-tank” senders, which means that you have to drill a hole into the top of the tank, but the technology is much more reliable and foolproof than the “through-the-tank” systems. They also don’t use any power at all until you hit the button to check the level.


  16. max

    does anyone know if there is any regulations for black/grey water holding tanks. I have found that they tend to be unrealistically expensive for being nothing more than a plastic bucket and i want to manufacture my own line. please let me know the referances as well. Thank you.

  17. madsci

    I’m working on my own tank level monitor system now, because I didn’t feel like paying $500 for a SeeLevel system and I like tinkering with this stuff. I might wind up spending that much in parts by the time I’m done, but it’ll be a more accurate system.

    I’m using hydrostatic pressure sensors for level monitoring. On the bench it’s working great for the fresh water tank, with 0.5% accuracy or better. Gray water’s giving me trouble because the system seems to get air trapped in it. Still, beating the 8-level resolution of the SeeLevel should be no problem even if I don’t get that fixed.

    The black water tank is simpler, as far as plumbing goes, but I may need to do some experimentation to determine the average specific gravity of the contents. I’m not looking forward to that part.

    My new panel will also have voltage and current measurement for the battery (bidirectional, aiming for 100 mA resolution or better) and for the solar panels and (currently unused) wind turbine. I don’t know yet how sophisticated I want to try to make the battery charge state monitoring - that gets tricky, but there’s a lot of literature on it.

    Anyway, I’ll publish details when it’s all done, hopefully by the end of the summer.

  18. alpenliter1

    I did this mod and installation, but have found that the tank readouts read each other. In other words the black water button might read the gray water and vice versa. Has anyone else found this to be true and what is the solution?

  19. Fred

    What does “Open Circuit” mean on the Seelevel I Model 707? Indicator lights for Fresh, Grey, and Black are displaying Red in the E and F positions. LPG is reading fine.

  20. tomharg

    I have done this mod on two different trailers. The first was done in 2009 and the second on a new trailer in 2012. The company now has two different sizes of sensor, a shorter one for smaller tanks like most of mine, and a longer one for larger tanks such as my fresh tank. Both strips now seem to indicate in 5% increments.

    The sensor strips rarely “just fit” the sides of the tanks. I have installed mine so that the sensors for the waste tanks are closer to the tops of the tanks so I have more accurate information as the tanks approach full, and the sensor for the fresh tank is closer to the bottom because I really need to know when that one is approaching empty.

    This system has worked very well for me on both trailers and I will buy it again for my next rig.

  21. Jim

    I put the seelevel sensors on our coach a couple years ago…
    Worked well for a while, then went bonkers, mostly after I washed out my wet bay area (sprayed everything with 409, then hosed the entire compartment off.

    Called seelevel, they said the sensor boards were not made to get wet and refused to warrantee their product.

    I found a system, new unused from a forum member, and replaced all 3 sensors. No help. I’d thought the problem might be the panel in the wetbay, but disconnecting it made no difference.

    Propane and battery read good.
    Fresh tank reads ok (I think)
    Gray reads “irregular” when less than half full, it will read different every time I push the button.
    Black seems to read ok on the “Top half” but reads 35% when I know the tank is empty.

    And the 2 panels do not always read the same.

    I used the original wiring, including the original power feeds. The inside and outside panels do not have the same power feed, that is how Monaco did it. All connections are soldered and heatshrinked.

    Any help, tips, or info on a better system is welcome.

  22. JLodge


    That’s too bad that the system went haywire - it isn’t a cheap fix. The dilemma that I see for you is that if you do get around to replacing it with a brand new seelevel system, you likely will run into the same problem time and time again. Although you could try to create a watertight case for the boards, there is no guarantee that it will work completely, and that might mess with the radio signal on the board. I would just suggest trying an alternative board that is considerably cheaper so that it doesn’t take quite a hole out of your pocket if it doesn’t work out. A similar water level sensor can be found here:
    This also is able to sense water over a spectrum, telling you the exact level of water. Also, it happens to be in a watertight enclosure: which hopefully may be useful to you.

    You could try other systems as well, such as a float based system or one that employs something resistively.

    Hope this helps!

  23. cooperrobt

    Nice mod but quite frankly the price seems awfully high.

  24. David Krueger

    My biggest problem is knowing my fresh water. I will never fill my gray or black tanks but I need to know my fresh water. I use the system. It connects to the water pump that is usually accessible and you don’t need to access the tanks. It lets me know exactly the water I have used and what is left in my tank so I never worry about running out of water.

  25. Elihu Leon

    is it possible to find a monitor panel for 1977 Ideal travel trailer? Ifso where

  26. Paul Shutes

    I just installed a used fresh water tank and the wires from the monitor are different colors than to ones on the tank. Anyone know about the proper wiring for a fresh water tank? I put all “spade connections” on it so I can swap them around but not getting any results. with the amount of water I have in it.

  27. kerry rogers

    I bought a micro-monitor to add to my forestriver sandpiper for gate discharge convenience. It will be a second set of tank monitor.
    Can u possibly send me a schematic on how to wire this monitor to the stock monitor that my sandpiper came with?
    Kerry rogers

2 Trackbacks For This Post

  1. RV Holding Tank Monitors :

    [...] By doing this mod, there will be no more guessing as to how much fresh water is left or how full the waste tanks actually are. And after a few times going through filling and emptying your tanks, you’ll get a good feel for what your water usage is and how you can better manage your holding tanks. For a more detailed article and some pictures showing installation, visit this mod on [...]

  2. Motorhome electrical/ battery upgrade questions - Page 4 - Pirate4x4.Com : 4x4 and Off-Road Forum

    [...] levels that could be used to drive a gauge. I wonder if the SeeLevel gauges work on LPG? RV Holding Tank Monitor System | ModMyRV __________________ FToy 086 - Motorhome [...]

Let's hear your comments about this mod!

If you are not a member, please consider registering so you don't have to fill out this form each time you comment.

Subscribe to the ModMyRV Comment Feed