On those years of Airstreams there was a generator option, which was a small battery charging generator, built into a compartment in the side. I think it was propane powered. I only ever saw one once, so my memory is fuzzy. It was a rare option, and some previous owner may have removed it, but the switch to start it remains.
Look again at all the outside compartments. It may still be there, or you may find one with some strange wires or holes where it was.
Let us know what you find.