Black tank leak

[Allesfam]

Water leaks from the top of my black tank when I turn on the flusher. This is our 3rd season with this camper but I'm thinking this has been happening since new. There was no way to locate this during the PDI without removing the plastic under-belly cover. When I noticed the liner sagging recently, I had no choice but to cut out a large section to inspect. What I found is water runs down from the top of the black tank when the flusher is turned on. The dump valve is open so it isn't pressurized or overflowing. I can only assume that the spray from the flusher is finding a crack or gap in the top of the tank to escape from. The black tank is mounted very securely and the toilet flange is glued in so I can't drop the tank easily to get a good look. It is not leaking where the flusher enters the tank because that area is exposed and no water is coming from there. I question why they glued the flange into the tank. Wouldn't a rubber gasket be better? That way the flange could slide into the tank and it would be more forgiving for slight movement or vibration. It would also be easier to remove or replace. Did they also glue the vent tube and the rear sink drain into the black tank? (While I was under there I discovered that the exterior kitchen sink empties into the black tank, instead of the gray). If they glued them, all of these pipes must be cut off to drop the tank. I did try to remove the toilet flange by removing the screws that mount it to the floor and gently lifting up on it. That is when I learned that it is glued in. Unfortunately, it doesn't seem to be glued all the way around. It moves up and down slightly on the one side but not the other so I'm thinking that the side that isn't glued so well is the source of my leak. I talked to my dealer and they are going to contact Keystone but I'm not counting on them covering this because I am outside the warranty period. Has anyone else had a problem like this? Anyone ever replaced the toilet flange? What is the best way to seal up the plastic underbelly cover once you have cut into it?