I've heard of water tanks that draw down the water level to about 1/2 or 2/3, but not completely empty. The draw down I had on mine was caused by a siphon like process between the air intake and the water overflow. Jayco approved a fix for this that my dealer performed. So far that's working well.
But again, it never drained the tank (and shouldn't be able to since both these ports on on the upper side of the tank).
2018 Eagle HT28.5RSTS
2017 Ram 3500 Cummins