Two 12v Deepcycle lead acid batteries (~$100x2) plus 200w of solar (Amazon: ~$280) and you will probably be able to boondock comfortably 95% of the time with nothing else. If you are running the furnace at night, it will suck a lot of those AmpHours. If the batteries get low, you've got the generator (or alternator) to charge them back up again along with the solar. If over several trips you notice the solar not keeping up, add another 100w panel (simple at that point) for another $100 to give you 50% more solar charging power.
Good luck!
Chris