underbelly leaking possibly from grey or black tank

You could put some dye in one tank or the other to localize the leak. Probably the gray tank. Maybe food dye would do it. Then you'll have to cut a flap in the coreplast and do a temp patch on the leak with eternabond if you have any. Close the flap up with gorilla tape. You may have to pull out wet insulation. Get some rubber gloves.