Adding extra batteries - where do I put them?

So another question comes up. We decided this weekend would be a good test run for boondocking with no changes and sort of figure out what we think we would really need if we choose to do it more often. We decided for this weekend we would take a cooler and not run the refrigerator. That should be our biggest amp draw. Aside from occasional lights, deploying slides and jacks, and perhaps the furnace a bit (shouldn't really need that), and the water pump, I am thinking the solar should keep the batts charged enough to sustain 2 days. I know all of this comes down to amp hours but I am not nearly versed enough yet to figure that out. But - I do like to think in terms of a worse case scenario. Suppose Sunday comes and we don't have enough juice to pull the slides in and jacks? Does the plug to the truck charge the batteries? If I hooked that up while running would it provide enough to prep to tow? And if so how long would that take? I was told the cord does keep batts charged while in travel but I never tested it and to what extent.

I know it's an elusive question lacking a lot of details but any thoughts are appreciated.

I really think if we decide to boondock a lot, a 3k genny or a pair of 2000's is a minimum requirement.