Chris, your lucky your knees still bend and are able to do this work.
I agree, I think you are right on all counts and i also think the wheel well repair would be easier and a lot less expensive than the tank removal.
I would take it to an auto body shop. I have had friends in the body shop business and they repaired rvs often. They fabricated things in their sleep, its less expensive than buying and waiting for parts sometimes.
Sheet metal or fiberglass will work. Try to save as much old material in one piece to use as a template. Take pics to show them before you rip into it. It will all end up being better than it was .
