I had the same problem with my 07 KZ Montego Bay 5th wheel (apparently that is a common problem for that unit) and researched the issue a great deal. I was lossing a 1/4 plus of my tank just by driving a few hundred miles. Wife was not happy to run out of water in the middle of a shower.
What I ended up doing was placing a shut off valve in the overflow tube.THEN, so as to not try and fill the tank or use it with the valve shut, I put labels on both the outside fill port AND on the pump switch inside the RV. The only other option I came across in my research, was to do as already suggested, extend the overflow hose lenght and mount it higher.