Water keeps draining out of the overflow

bdpreece wrote:
Unfortunately on many rv's the overflow is plumbed in such a way that once it starts to drain for any reason such as sloshing as you go around a corner, it creates a vacuum and can empty the entire tank. For now your best bet is to plug the drain but don't forget to remove the plug when filling the tank. The correct fix is to find the overflow line and loop it upwards above the tank then down.

What bdpreece said ^^^^
This is a very common issue. I see trailers(including mine) and 5vers mostly doing it while driving down the highway. I suspect because there is little clearance between the top of the tank and the floor above it. Definitely search for the low point drain shutoff first. However, if it turns out to be the vent valves one common solution is to put shutoff valves on the tubes. Then close the valves while traveling. Leave them open the rest of the time.