You do not need to have the largest generator if you are only plan to charge your batteries. You need a larger one if you want or need to run your A/C. I have a Champion generator that cost me around $500US dual fuel and it is more than enough to run the A/C while charging the batteries. I have never put gas in it, only run it off propane. Most places I boondock have no restrictions, but if anyone is near, I don't run it late at night or early in the AM. I usually only run it occasionally if it is really hot or if we want to watch a movie, the battery charging is a bonus.
When I got my 5th wheel it had 2 group 24 batteries. That served well until I needed new batteries. When I needed new batteries I went with the 6vGC batteries from Batteries plus Bulbs - around $300 for both. The 2 6V definitely last longer than the group 24. But it is what everyone has said, depends on how you use your batteries. When I dry camp, I think of it like tent camping. Only use the lights, power, etc when I really need them. If my wife would follow that we could go longer without charging. Can't tell you how many times I go inside the camper and a light is on. Anyway, we run out of water or waste storage before power.
Solar sounds great, but it's expensive, doesn't allow you to run your AC equipment like microwave and A/C, and doesn't charge well if you park under trees, and doesn't help if you use your heat and the solar system isn't large enough to recharge your batteries with the available sun the next day. If you have to have a generator for backup, skip the solar and just get a generator - unless you are a full timer or plan to spend weeks in the sun.
