It sounds like you've given this a lot of thought. The most efficient way is going to be swapping the batteries. Just make the battery location as easy to get at as possible.
I certainly wouldn't go the inverter route because you'll be wasting a lot of that power going from DC to AC and back to DC again.
You also won't get much of a charge hooking a full battery to a low battery.
Swapping is the most efficient (and most difficult) way, but you'll get the most use of the power you have. Driving around for an hour probably won't completely top off the battery either, but the longer you drive the better off you'll be.
