RV salvage yards? There is a list of them in this forums RESOURCES section, or you can Google 'rv salvage'.
Also, the fuel tank will be one of the standard ones for the largest model '79 GMC van, so maybe find one in a regular salvage yard or a new one from an aftermarket tank source. A class C rv is built on a van cutaway chassis, so the tank will be from GMC rather than the RV builder (Coachmen).