Too add to the above by JK, you might consider taking a generator or a install solar panels to keep your batteries charged. Like JK states, the furnace fan is a power hog that will deplete your batteries fast.
If you plan to dry camp often, two 6 volt golf cart batteries will last longer than two 12 volt batteries.
2015 White Hawk 20MRB
2011 F150 Non EcoBoost (5.0)