[hiccup]

All of us w/freshwater tanks built with so little forethought as far as replacement feel your pain.

I don't have a solution for you, but I have considered this as I had a leak in a fitting that I thought briefly might be the tank.

I know they make epoxy kits that are designed to seal fuel tanks from the inside. My friend used this on a motorcycle a long time ago. You pour the epoxy in and rotate the tank around to coat the entire inside. Obviously, you can't do this with the freshwater tank. However, I wonder if you could cut a large access port on the top of the tank, completely dry the inside of the tank, and spray something similar to coat the inside of the tank. For what it's worth the motorcycle gas tank had several pin holes that had rusted through but after the coating it was leak free for as long as he owned the bike (at least 5 years).

I don't really think the above is a great idea but figured I would share because I'm not sure there is a great solution to this problem.