The 7 blade connector has wires running to the running/tail lights, brakes, left brake/turn, right brake/turn, reverse lights, a fat wire to the house battery+, and a fat negative wire to the battery - or frame.
You should note that the wiring standard used in 1974 Airstreams is different from the current one, so you have to do a little umbilical customization to get things to work if your umbilical has not already been rewired.
My
Airstream Umbilical Replacement blog post might be helpful.