Jon, later model Airstreams use the main batteries, beginning probably about  1968-1970. We have had 72 and a 73 all of which had the breakaway switch wired into the on-board 12v system. In
our case, the 72's had an electric which made it quite convenient. Sometimes sensible things happen! Betcha though that some 'streamer thought it up, wrote it up in Trailer Life, and someone at the
Company took the idea to the boss! Too sensible for the factory to do it.

Donald