Step one may be to try finding a label near the driver's left side. If that label is still there, it should have the info on what chassis the RV was built on. Think of RV as a house built on top of a chassis which was produced for many different type vehicles.
If they are able to find what chassis is listed, they can then begin to search salvage yards for anybody having a chassis which still has a good radiator. Obviously this is going to be a real challenge, but there are many salvage yards who do still have that age vehicles on hand and they also are likely to have networks where they can post items they are wanting!
I've not used a salvage yard in many years but they had networks in the 70's for parts, so I would assume it natural to have far better search systems now.
One might also begin a search by contacting one of the RV salvage yards that are spread out around the country? Ask if they have a way to find one?
But being real about the situation, the odds are slim. Radiators are prone to corrosion and easy to damage. They are also among the first things that get destroyed when we run into things!
