Fresh water problem with water overflow while being towed

Yep, happens all the time. Sometimes I have followed our fifth wheel with in my car (because I might have to go to work) and can see the water come out of the overflow as he takes turns. However, ours at max will be down 1/3 if that. And the dummy at the RV place who said most work off of city water. NOT... many of us boondock, are at State parks without direct hook ups and use our fresh water tank more than not.