I've been doing an online search for the appropriate fuel tanks for you but am discovering they are real tough to find online.
If you don't mind salvage, an RV salvage yard should have something compatible. A good nationwide list is:
http://www.ccis.com/home/mnemeth/rvsalvage.htm (wouldn't hurt to ask if they know of sources for *new* fuel tanks. They just might).
Depending on the tank's overall condition maybe a simple de-rusting, cleaning, and repair will be enough. Both plastic and metal ones are usually repairable and it might be cheaper to go that route. Welders are in the phone book, or look under "plastics" or ask the motorhome mechanics.
You might also call up your local RV supply store and and the motorhome dealers and ask around. Most dealerships have a service department that can do parts locating. Your local motorhome repair shop should also be able to locate some leads for you. All these places are listed in the phone book. Call around, persist, and you'll get the answers you need.
Hope this helps,
--Boumboum34