Fresh water overflow?

If you are losing water out the vents while traveling, it is not a siphon, it is simply a spill that occurs while rounding a curve or driving up or down hills. Many thousands of RVs have been built with the vent pipes attached at the top of the tank and then go straight down and out the bottom. Of course when the "bucket" is tilted, water spills out. The proper fix is to run the vents out the side well above tank water level. Putting valves on vent pipes is a dangerous idea, one time doing it wrong will cause severe damage. Notice that I said "pipes" as in two or more, if there is only one then that too is wrong because the venting should have twice the capacity as the supply line.

The pickup point for water supplied to the pump should be on the bottom, many are on the side which doesn't allow full use of the tank capacity. The fix is to plug that port and attach the hose to the drain port with a tee.