[Image Removed by User][Image Removed by User]My heater air return had no filtering system.  I very easily added a “cut to fit” filter, cutting it to just a bit smaller than the grill.  Home Depot or Lowes has it.  I removed the grill, and mounted the filter behind the grill.  If you use the heater much, you should check/change the filter monthly.

The filter I used had a plastic “cut to fit” grill, which keeps the filtering media sandwiched between the inside & outside grill. 

This mod takes only minutes, and keeping dust & stuff out of any air handling system is good. 

 Hi, wish we could have seen your pictures. My trailer has three heater vents, but I don't know where the intake is. I have a louvered grill in my bathroom, but when removed, it seems to be not connected to anything. Also the louver grill is very close to the bathroom heater vent, so I wouldn't think that would work too well.