Looking for parts info, just as a test of what might be found and I now see why they might have little to none on hand as that is from the old microfiche files and what they have online is such a poor copy that it is useless!
So what IS the plan for the old buggy? Is it hoped to be used as an RV or is it going into farm service like a hay wagon?
At that age, finding one that has a tank still in good enough shape in a salvage yard is going to be a bit of miracle but might be among the better choices if the OEM tank is actually needed.
I am sure that age group was built on a stripped chassis, so looking for the label might give you some more exact idea of what chassis to chase in salvage yard calls. Have you found that label near the driver's left side somewhere as it lists lots of specs if still able to read it.
going by chassis may open the market for wider searching as RV are often just built on the same chassis as lots of other vehicles like bread wagons, beer wagons or service trucks of some type.
If you can move to finding salvage chassis, rather than an RV, it may open the possibilities a lot more??