Never had this happen on a black water tank but I did have a gray water valve that the handle pulled out of it when I went to dump it. Only way to fix it was to take it apart. Sometimes you have to do what you got to do. It was a mess. Then I let someone use my travel trailer once when they had an older family member visiting. They shoved Depends in the toilet. I pulled up next to the dump and ran a rod up the valve. I pulled out a few old Depends and then the tank let go and******went everywhere. Took a while to hose that mess into the drain. Happy Trails! If I ain't DoneIT it ain't worth talking about.