I have an 2007 Winnebago Sightseer 35J. When i fill my water tank i shut the water off when it starts to come out the over flow. I noticed that the water seems to come out of the over flow for some time before it stops, when i arrive at the campground i could be has much as a third low. I decided to investigate whey this was happening, I filled the water tank for a camping trip this weekend and the same thing happened, If i lift my bed up i can see my water tank and as the water was being siphoned out after the tank was filled the tank was actually caving in. I opened the main drain and let some air come into the tank and break the siphon. There is a elbow that comes off the top of the tank some tubing is connected to it to drain the over flow water out the bottom of the motor home. When i removed the elbow there is no other tubing that goes into the tank. I'm baffled as to how it can have such a strong siphon