If you have the old tank, take measurements and photos and search for custom plastic tanks on the internet. Companies like Dura Weld can make a tank for you. If you don't have the old tank, plan where you need the fittings, draw what you need, then contact a tank maker. I'd be willing to bet there aren't any used black tanks available for a '72 Argosy. I had a tank made for an Argosy some years back and it was not overly expensive and better than the original with heavier plastic material and heavy duty fittings.
1966 Airstream Caravel
2006 Toyota Tacoma