But wait -- when water drains out of the
tank it must be replaced with air, so there is an opening at the top
someplace.)

The actual fill tube doubles as a vent, so yes, there is a big opening to allow air to get in. It may be possible that the utility fitting has a crack in it and it simply dripped down the van wall. The actual green colored fill tube is pretty stout as the clamp at the tank is pretty hefty. I believe it threads onto the back of the utility hookup, and if over filled it would possibly leak up top. It was suggested to me several years ago to put a piece of Styrofoam on the fill float (if you have that type) which increases its buoyancy. Not sure about the other type of fill sensor, but think they simply ground and they just may require some cleaning.

Troy