In my humble opinion, an A3 would make a great starting point for a camper. Granted you’re not going to get there fast, but who cares, you have your whole world with you. Cat and Allison parts are available world wide. And the platform is simple enough that a third world mechanic can fix it. (No offense intended) I think that 5k lbs is a reasonable starting point for an RV build. It will probably go overweight once loaded (water + fuel are heavy) but I think the chassis can handle it, even off-road. My A3 will go further than my stock 4runner will go, with the exception of wheelbase maneuverability. I think an A3 with lockers would go most anywhere else I needed to get with my house. Are you going to be able to wheel it like a short wheelbase, modified jeep? of course not. Are you really going to push the limits of a 100k$ unimog with your whole world on it rock crawling? Also I think that an 8 x 12 foot living area, with the bed over the cab, would be about the minimum for long term comfort.
Almost anything can be done with enough time, money, skills, and determination. Everything is a compromise......But for an economical, durable, realistic, starting point; I think the A3 is hard to beat, unless money is no object.