Since the tank only has to be sealed against gravity and not pressure, I would thing you could just clean it well and use one of the following: Goop, Gasket Sealer, JB Weld, Flex Seal, or any number of products.
Just before I purchased my van it was obvious someone replaced the gray dump valve. It was also apparent from the drip, drip, that they did not get gasket to seal properly. Since the valve was new and working perfectly, I bought a can of Flex Seal spray. After cleaning the area, I applied a light coat a few times a day over several days. No more leak.
Let us know what fix you finally come up with.