I wired in a voltage regulated relay and put a spare battery in the frunk. Bought the ResMed 12v adapter.
Battery charges while riding. The relay feeds the battery when voltage is above, uh, 13?
I tested it for two nights without charging and could have probably done three.
I love wiring, so this was a fun project. The ResMed piece was the single most expensive item..
Used it on a couple 7-9 day camping trips without a hitch.