The reason there was a separate charging line for the battery is twofold:
1. The original 6-way connector for the brakes and tail lights did not have a 12V
power line. New 7-way connectors do.
2. There were still some positive ground automobiles back then. With a separate charging cord you could rewire it to match the polarity of the tow vehicle.
Most people nowdays switch to a 7-way connector. The drawback is that you can't really get a useful amount of current from the tow vehicle to the battery this way, unless you are on the road several hours and do some careful engineering.
I wasn't long after your trailer was built that Airstream started installing 'Univolts'. These were large, noisy 12 volt
converters that could run the 12 volt
system and recharge the battery, off of 120Vac feed. Those are usually replaced nowdays with Intellipower converters or some other brand. Doing a forum search on Intellipower or converter will yeild several threads on how to upgrade your system.