Go On YOUTUBE
Same think happen to me, I went on Youtube. Easy fix by Entertainment if it is a Dometic. I just push handle to lower flapper, Removed Gasket and cleaned it, put Petroleum Jelly around it, and reinstalled. Two minute job. Took care of problem Make sure water is turned off.
We are driving a Dodge Durango and pulling a Evergreen G269FK. We also have two Cairn Terriers. 2 Champion Generators. This year, planning on 6 months in our Evergreen visiting, California, Michigan, and Ohio. We live in Florida. Grandkids, and dogs are why we camp. Also have a Hensley Hitch which that eliminates all sway. Pricey, but worth it.