Hmm, models that carry a tinny on top are usually side fold soft floor, though I guess you could adapt a system to suita rear fold hard floor, I just haven't seen any myself. Your situation is ironic as the mrs and I can actually see ourselves going for a full size van once the kids are off our hands.
But as you say there are many tight spots thata full size van has trouble with, and in our experience these spots are more often than not the holiday caravan park rather than the bush.
One thing that will be a revelation is the reduced weight hence reduced full use + ability to actually sray at speed limit up hills and even overtake. Good luck with the search.