Originally Posted by BigTJohn
Siphoning is the problem. Once it starts, hard to stop. I bet there is no hose to the back side of your water fill station, a small screened hole right above where you put the water fill hose into. I ran clear tubing (mine is 1/2" ID) using a plastic barbed union from the FW tank vent tube (1/2") to the back side of the water fill station. I needed 11' of tubing to do this. No real pressure to deal with so I just used tie wraps on all the connections. Now the vent is screened from bugs and the vent is above the level of the top of the tank allowing the "vacuum to break" stopping the siphon. Just traveling will allow (make) the FW tank to expand and collapse a little and while doing so the water being non-compressible will find a way out - through the vent tube.
Nope, no breather hole above the water fill, only the 'over flow' coming out of the bottom under cover next to the tank drain tube. The 'over flow' does not even seem to act like a breather tube. The filler to the FW is an attachment for a hose only, not an open port. The filler port has a one way check valve in it. This check valve seems to compound the problem, letting water/air in and forcing it out through the p-trap over flow tube. Hence...self draining.