Those generators were an unusual option only in the early '70's. As I recall they are 12 volt
only, designed to keep the batteries charged.
There have been some posts on them, Andy at Inland RV seemed to know and recall the most information. As I remember his posts, he said they were very altitude sensitive and had to be adjusted a lot to keep them running.
I did a quick search of the forums and had no luck in finding his posts.
Propane generators are fairly common, it is not an unusual fuel for them.