With some water tanks the fill tube and the overfill tube are not high enough above the tank. The water ends up sloshing up the tubes when you drive. I had that issue with a 27 foot motorhome. I put a plastic hose into a half gallon water bottle and plugged the other end into the fill line and the overfill line. Within a half mile and a full tank of water I had water in both bottles. The fill tube was by far the worst. I ended up putting in a full plug on the fill tube, and kept the hose into the bottle for the other line with ability for air to breathe off, but the sloshing remained a problem unless I only filled to 2/3 of a tank.