The best things I've bought are a Honda 2000W generator, 2 heavy duty deep cycle batteries and a gray water tote. A lot of the campgrounds here that don't have hookups will have water from a pump and a place to dump gray. I can survive for days/weeks with that setup. And the water thing is really only a big deal for me because I have 3 small kids. It's nice to be able to give them a bath every couple days.
In addition to what I have on board for water I also carry three 6 gallon jugs full. Last time I was out for a week I never even needed to go to the pump. The 40 on board plus the 18 in the 3 cans was more than enough.