Fresh water holding tank overflow HELP please

We have a similar issue. The overflow tube seems to create a suction if the tank is over-filled, if we park on a hill, or if we are driving on a curvy road. You can search for this issue on this site, and the solutions seem to be: Plug the overfill tube, run a loop above the overfill tube, or create a vent at the top of the overfill tube.

I'm tempted to run a loop above the overfill tube with a valve that I can close when traveling, and open when operating the system. . . if I can remember.