I would look at batteries first, assuming you have a single 12V battery now. A pair of golf cart batteries will give significantly more usable power and is about the most economical way to do that. If you have incandescent lights, replacing them with LEDs--or at least the ones that get much use--would also be a significant help at a pretty modest outlay.
Beyond that, there are many variables. If you're only talking a weekend or a few days at a time in moderate weather (so no significant furnace usage) you may not have to do anything more. If you'll be camping in a mountain forest, solar is less useful than if you're camping out in the open and so a generator may be a good option to charge the battery bank. Conversely, if you're in an area with lots of sun, solar is a great option and you may not need too extensive or expensive of a setup. For only very occasional use, it may be more economical to charge the battery via jumper cables from your tow vehicle (of course having it running during the process).