Some time ago, I would have chosen Earthroamer, except that they're now too big. They used to offer a model on the Ford E450 4X4 truck standard cab PU - which was a more reasonable size - but I don't think they will build on that size chassis any longer.
Now my cost-is-no-object choice for North American camp-anywhere RV would be either a fully loaded Tiger pop-top on a SWR one-ton 4X4 standard cab chassis, or a fully loaded Sportsmobile pop-top on a SWR one-ton 4X4 standard length van chassis. There's not much room in these, but they can be equipped to provide four-season full comfort once you get where you're going just about anywhere in North America.
It would be a real challenge with an unlimited budget to stay small - but if you want to go "anywhere" in North America - you would have to constrain yourself on raw size. I've noticed, on our "relatively small Class C" RV on a ton-and-a-half chassis, that there is A LOT of wasted space everywhere inside it and up underneath it all around the frame. I'm fairly certain that Tiger and Sportsmobile could put together an over-the-top really well equipped ultra-small custom model ... given a blank check.
By the way, the object of the ultimate RV is not to get to an ultimate destination in rich-man style ... it is to travel no place in particular in rich-man style. Rich folks seldom desire to travel no place in particular - and that's one of the curses of being rich.
