Re: what van?
As I understand it, most of these are built using 'normal' vans. Ford, Chevy and Dodge at least used to be the primary choices. Check out the cab for driver comfort. There is one (I think it was Chevy) which had very narrow floor space for the driver. My dad had a Dodge and I had an old Ford which weren't too bad for foot comfort.
I think I saw where one of those companies stopped making vans, but I could be misunderstanding the article I read, since this move doesn't seem to make good business sense. Also, I've heard about a new chassis which I think is made by Mercedes which might be of interest. Best is to test drive the motorhome before contracting to buy, but you might get some info by test driving samples of the 'normal' van of that brand.
I had a older Ford van and it gave me nothing but trouble. The front suspension was very strong, as long as the wheels were straight ahead. If they were turned, any little bump could knock it out of allignment. And the way they alligned it was to bend things. Basically it was unreliable and not pleasant to drive. Plus Ford is trying to force social change unrelated to the automotive business, which I object to on principal.
If it was me, I'd see if Chevy had anything comfortable to drive, and especially check out the Dodges. Also, that new chassis. Plus, I would see what Sportsmobile could do for me, since they allow you to design your own floorplan in any van you want.
Check out the web sites for the various manufacturers, they ought to have the floorplans available.