A very good option, especially for RVs with unusual tank designs is a system called Gas Tank Renu, which I believe is a franchised product. A dealer will remove and clean the tank, inside and out, before coating both sides with a patented product. The result is your tank was 'repaired' and is now plastic coated, and an exact fit. It also has a lifetime, nationwide warranty. The process is priced fairly, so that many vehicles, even those with replacement tanks readily available, could be coated for a lesser cost.
We have a radiator company here in Albany, NY who specializes in sealing gas tanks from the inside out. I am sure if they could seal his existing tank it would be better and perhaps less costly than having a new one fabricated. They have been resealing leaking gas tanks for quite some time. In fact they did my father-in-law’s 1975 Chevrolet Caprice back in the 80's with no problem.
Have Phil find a shop that restores old cars. They will take his tank and clean out the inside, pour a plastic compound inside and roll it around to coat the inside all over and pour out the excess and let it cure for a couple days. Then they can spray the outside with a black paint that double seals the tank. Then the tank is better that new and looks like the day it was first installed. People who restore old vehicles find the tanks are always filled with pin holes and they want it to look original. I have sealed three tanks now and have had good results. It will cost him at least a hundred dollars, but it’s worth it, when he junks his rig that tank will still be without a leak.
So there you go! Thanks gang, RV Doc