It's most likely siphoning from the tank from the overflow drain. If you add a loop, like a trap in the line, the siphoning will stop.
Whatever you do, DO NOT block the overflow vent. The coach will literally explode (ok - a little dramatic) if you fill the tank and the air has no where to go.
We just finished repair to a coach where the owner blocked the overflow drain for the same reason -- about $18K later.... Bent all the steel framing in the floor, slides, broke the kitchen side walls, counter, flooring, it goes on....
P.S. It wasn't a Winnie.
'07 Winnebago Journey 34H - CAT C7, Koni's, MCU's, SS Bell Crank, Safe-T-Plus
'07 HHR Toad, SMI AFO, Blue OX