I've seen one done on the back of an old farm truck but not on an old motor home.
Two issues I see would be all the nasty demo work and waste created depending on how old/nasty the motor home was, then there is the weight issue. Motor homes tend to be built light weight for the square footage compared to a tiny house. Be sure the frame, suspension, axles, tires, brakes can take how much weight you would be putting on compared to the original light weight RV style structure.
Two options I would suggest for keeping the weight down would be to use vinyl an/or thin metal siding on the outside instead of
wood siding and if you do use wood go with thin cedar. On the inside using 5mm vinear ply is a beautiful, strong, lightweight, & reasonably low cost alternative compared to the 3/4" thick tongue and groove wood often used for the inner wall cladding on tiny houses and of
course is far lighter then any Sheetrock type inside finish.
I personally really loved some beautiful sheets of the 5mm ply vinear I just used to make some light weight but strong drawer units. It was absolutely beautiful rosewood color on one side and beautiful white wood on the other side hardly weighed anything and I could easily pick up like four 4'x8' sheets at a time and it was only as heavy as a single sheet of regular 3/4" thick plywood and so strong when properly glued & screwed. And it was slightly cheaper per sheet then the cheapest junk OSB board.
In fact unless you directly screw metal siding to the studs and use the metal for the external shear (Not a bad idea either for both cost and weight savings provided you can find the right kind of metal siding to make this work for the right price and still look classy as well) I would strongly suggest using the 5mm ply vinear as the exterior sheeting under the siding rather then OSB or normal thicker plywood both for weight and cost reasons.
