Nearly all RV's are based on a third party platform.

* Class B - what you mentioned

* Class C - what you mentioned plus GMC vans and medium-duty Ford/Chevy/RAM trucks.

* Class A - Commercial bus and truck chassises. Freightliner, International, etc

No reason to build your own platform from the ground up the major brands have service centers nationwide.

Aside: I'm a bit unclear on RV classes. Turns out that they're not arranged in strict order:

- Class A: 20'--40', largest, up to 30,000 lb GVW.

- Class B: smallest, 17'--23' with the VW Bus camper van being the original (according to the source below). Think "camper van".

- Class C: Intermediate, 20'--35' length, usually on a van chassis with an over-the-cab design (usually integrating a bed).

There's also a range of trailers and camper-shell designs.

I believe that different drivers licence certifications may be required for some classes (particularly Class A).

<https://rvshare.com/blog/rv-classes/>

<https://rvrentalscout.com/rv-classes/>

<https://en.wikipedia.org/wiki/Recreational_vehicle>

I'm not really sure if there's an official standard. Generally, I've seen:

* Class B - Essentially built within the confines of a commercial cargo van.

* Class C - Built on a cab-over or chassis of a medium duty truck/van.

* Class A - Built on a heavy-duty bus chassis.

Now is a good time to interject a lament that VW has not brought the California/Grand California to North America. Given that people buy $150k vans from garage upfitters like they're going out of style, I think VW would absolutely clean up in this market, and could hit a significantly lower price point (California starts at GBP60k, Grand California at GBP80k in the UK)