There should be a vent, they all have one.
Mine, although a different model has one vent in the garage on the wall from the living area.
As for the water tanks, I've not heard of this problem before. But there are usually one way valves on the fill lines, and many of times those have been installed backwards. Might be worth having them check that.
2015 XLR Nitro 39DKS5 Toy Hauler
Towed by 2016 Ford F350 CCLB DRW 6.7