Fresh water loss while driving

I'd be surprised that you get much out the overflow / vent pipe. They usually mount them a little ways above the tank to prevent just that from happening.

If though you are getting spillage, I would definitely suggest a simple floating ball check valve over a standard ball valve. First because it's automated so there's nothing to forget to do. More importantly though because to eliminates the possibility of damage to the plumbing in the RV.

A floating ball check valve is a little device that looks a lot like an inline fuel filter. Inside the body of it is a little hollow plastic ball that gravity drops down into a recess to mostly close off the opening, enough air can pass to allow for normal operation though. When however water comes up the line the ball floats, rises into an upper recess and again mostly blocks the opening. Enough water can come out to tell you the tank is full, not enough to cause an issue.

I wonder though if the real issue isn't that your RV, like mine, has twin water tanks. In my case one under each dinette bench. There's a crossover line about 3/8" between the two to balance them out. However that little line underno pressure save the water weight itself is extremely slow to equalize the tanks. If I fill the water with a hose and stop when water appears I have one tank full and one tank 1/4 full. If I leave it sit for a while and recheck, I have a reading of 1/2 a tank of water because it's now divided between the two.

I have 1/2" ball valves valves on my vent and overflow lines. I close them if I am traveling a long distance and concerned about loosing too much water

What is going on is NOT a siphon, just a simple spill. The vent pipes are attached at the top of the tank and every time water sloshes that way it spills out the pipes. You can lose most of the water on a curvy road. The only real fix is to run the vents uphill and out of the coach well above the water level. The old gravity fill type had no problem because the vent was above water level. Read this months,(Jan 2014)Trailer Life for more information.

Tank has created a siphon effect from filling.........due to lack of proper venting

Check the vent line routing.
Straight up as much as possible from tank, no loops, no kinks, no trapped water

When I had the Triple E the fill tube was inside a carpeted compartment.. I finally figured it out by putting in a wine carboy tapered plug into the fill tube, and fed that into a half gallon jug. The overfill tube got the same treatment.

Within a quarter mile of turns, the fill tube had spilled enough to almost fill the jug, and the overfill tube has spilled about 1/8 that amount.

After that I devised a way to cap the fill tube tightly, and I just ran with the overfill tube into a jug with the hose able to breathe free air. I was then able to fill my tanks and run for the campground without worry.

Mine on the last TT I had did this and I thought of putting an on off valve inline of the pressure or over flow tube but I had second thoughts, I was afraid I would forget and blow the tank apart. so I just filled up before I left the last town and I also carried some extra 5 gallon jugs to make sure.

Are you losing more than a gallon of water? Otherwise, I'd just leave it alone!

WoodGlue