I have the same issue with my Class A motorhome. The overflow/vent outlet is poorly located on the side. I lose a lot of water in transit, when I make a left turn and when I am on an incline that makes the water flow to the passenger side. I modified the routing of the overflow/vent line. It helped some, but I still lose too much water. I think what I really need to do is take the tank out and relocate the overflow/vent outlet to the top-center of the tank. I've got to do a little research, but it doesn't look like it will be that difficult.
I've attached my before and after pics.
