The Chevy was a body on frame design. A RV upfitter could punch holes almost anywhere and not compromise the vehicle's structure. Drop the floor? No problem. Cut storage compartments in? Works. Yank off the back and add a fiberglass structure like the PW Excel? A-OK. However, the Euro-vans are unibody construction, and it takes a lot more work to cut panels and still maintain the structure of the vehicle.
I have mentioned about engineering hurdles with the PM. Though not impossible to surmount, as most European "B"s are PMs, they are still something to engineer around. I have wondered if tank placement would be made easier if macerator toilets were used, so the black tank can be positioned in the best location, and the toilet doesn't have to be directly above it.
Of course, the advantage of the PM is its width. Most people under six feet can sleep transversely in the van. However, the US PM is shipped with FWD only, so upfitters have to deal with a very low van clearance. If Fiat could ship a 4WD model that sits a couple inches higher, this would make upfitting a lot easier.