It sounds like are camping styles are somewhat similar. Most of my camping is in national forests and I opted for a 16’ to get as far back in as possible. And I graduated from a PU, too. Coming from a PU I imagine you’re pretty frugal with your lights and water.
My thoughts would be stick with your sub 20’ idea if you want to get well off the beaten path. The biggest tanks you can get, LED lights, and as much ground clearance as possible would all be advantageous, too. Also two true deep cycle batteries. I use one group 27 marine battery and can manage to get the best part of a week out of it. Generators are an option, but I like to camp in natural setting and motors running seem out of place there. So I don’t have one. I camp in the forest so solar isn’t a good option for me or I’d have it. If you have the option of open meadows it might work for you, though.
I carry extra water in 5 gallon cans if I’m boondocking and just refill as needed. In a cg with water I just fill there and carry it back to the TT and refill. It’s just a matter of having something to funnel the water in the fresh water tank, basically a two person job with one holding the funnel. As far as dumping goes, just pay attention to what’s in the area you’re camping in and you’re almost sure to find a dump site. You may have to pay a small fee, but I don’t recall ever having to do that.