I do a lot of 2-3 day dry camping trips. I think the best bang for the buck is to keep the old batteries that you took out (assuming they were still good) and to use them as reserve. This is what I do- I upgraded to two 2GC batteries but kept my old pair of GP24's and boxes. If I'm taking a trip that I need an extra day's use out of, I charge the GP24's at home and stick them in the back of my tow vehicle in the boxes and strap them down so they don't spill. Then when the main bank of 6V batteries gets too depleted for my tastes I disconnect them, pull them off the tongue, and replace them with one or both of the GP24's, remembering to wire them in parallel instead of series. Whamo, full batteries.
All this cost me was not getting the core charge back from the GP24's. I might wire in an extra battery box under the hood of my TV so I can charge the GP24's while driving, but so far I haven't needed this. I don't bother to take them with me when I'm going someplace that has hookup or just for 2 nights. At home, I just store them in a shed with a battery tender on them and check the water once in a while.
An extra set of batteries weighs about as much as a generator but it's much cheaper to replace if it gets lost or stolen. When the GP24's wear out I'll get another pair of 2GC's and put them in the rotation.
