Many shuttle buses are made by the same companies building Class C and Class A RVs, using similar shell construction technology but without as much insulation and no provisions of wiring and plumbing, no reservation of space for tankage and storage areas.
Shuttles may have somewhat stronger floors than the lightest motorhomes, and small transit buses furnished with heavy seats and seat belts will have even stronger floor structures. When equipped with air conditioning, they usually have much larger A/C units that RVs get, because of the higher heat transfers through windows and uninsulated walls, but often use of the A/C requires the engine to be running.
Considering how cheaply used shuttles, small transits, small school buses and small panel vans can be bought, some type of conversion can usually be done for less than the cost of a new RV, or maybe even an equivalent age used RV, if you consider only materials and not the value of the builder's time. I've seen conversions done on all these vehicles, as well as small box trucks (U-Hauls are popular for their low floors). I know someone working on a conversion of an ex-military 2 1/2 ton 6x6 shop truck, so at least he is starting with an insulated box with a ventilation system, but he will be handicapped by a chassis really unsuitable for highway use.
Most of these motorized RV conversion platform options will be high mileage when they come on the market for resale.