I plug the camper in at home a day or two before we leave to give it a good charge and get the frige cold.  This will get me through a long weekend.  When I was out in Yellowstone for 7 days I got about 4 days out of the battery because of using  the furnace.  I made a cable up that was 7 prong and fit to my hitch plug and the other end to the battery in the bed of my truck.  It would charge while we drove around all day.  It would give me enough of a charge to get through the night and do it again the next day.