Your comments about the cost interested me enough to do a little math
If we assume, say $50,000 for a modified 12'6" high, 96" wide, and 40' long semi-trailer (complete with add-on basements and slide-outs)and then, also assume that the finishing costs (adding the interior and exterior hardware, (carpet, appliances, cabinets, beds, awnings, holding tanks, etc.) would be more than a custom home . . . let's say $125/sf, the total cost would be approximately $100,000 ($50,000 + ($399 x $125).
Add the cost of a nicely outfitted VN610 and your at $135,000. Certainly not the cost of a pop-up, but then, when you get into this league, the measure for comparison changes. As you said, a Prevost might run $500K, so a "tractor/trailer camper" seems like a bargain to me!