At a lower price level, Winnebago has a specialty vehicles division that builds mobile offices, command posts, mobile clinics, and accessible motorhomes. The division can build whatever will fit one of a number of different shells they use. When I was looking at them a few years ago, they were building mostly on entry level A-gasser shells, and non-slide Cs, for accessible RVs.
At the other end of the scale, there is Newell in Miami, Oklahoma. Every build is a custom design. AFAIK, everything building now is on their big 45' platform, quite expensive, legendary quality.
Then there are the coach converters outside the RV industry, producing bunkhouse designs of many different capacities for the entertainment industry, and other businesses that require employees to sleep in transit. Not all Prevost, converters work also with other shells and chassis available in North America.
Similarly, almost every build on a heavy duty straight truck chassis is custom, to the initial buyers specific needs. We've had chemistry labs, seismic acquisition and processing centers built on HDTs, in addition to field housing for employees. Hardly ever put something into a motorcoach shell or onto a RV chassis. Sometimes what you need for a life on the road is outside the box defined as RV industry.