I agree with Kulea! His rumination seems correct to me.
I remember the VW Campers had a dual battery option where a second battery powered all the camper lighting, etc, but was not connected to the vehicle electrical system until you started it at which point a relay controlled by the K? (switched ignition) circuit turned the relay on. (when cranking, this circuit wan't powered. Headlamps and several major consumers were on it to provide more available power for starting. This also had the benefit of de-coupling the second battery so the high current of the starter wouldn't fry the relatively small wiring to the aux battery)
As long as your vehicle has such a "load reduction" circuit, implementation would be relatively simple.