Mine does it also but only after I fill it on my slopped drive and then it is probably on the oposite end. I would think a valve would be risky. What if you fill and for get to remove it and blow a hose out or something else. Are you boondocking that you are loosing enought that you running out. You could Add hose and find a place to make a high loop that make cut down on sloshing issue.
07, Cherokee, 295 5thwheel