I'm kind of confused about the concept of a water vent siphoning a tank dry.
In order for the vent to 'siphon the tank dry' there would have to be two things that I've NEVER seen on an RV.
First, the vent tube connected to the water tank would have to extend to the very bottom of the tank. They are merely attached to a nipple on the top of the tank, there is no internal component at all.
Second, the vent tube would need to extend to a point lower than the lowest point of the tank. I've never seen a vent that was even as low as the top of the tank, let alone the bottom.