It seems to me it's a situation similar to IH and other bus body manufacturers. IH built the chassis/engine, and it was shipped to American Coach/Fleetwood as an unfinished vehicle. Once it left the second factory, just like a Blue Bird or any other brand of bus, it is considered a finished vehicle. The second factory issues a Certificate of Origin (IH may have done this for the chassis as well; the second factory either keeps it or submits it with their own); this certifies the vehicle's weight, wheelbase, GVWR, and that the vehicle meets all relevant safety and emission rules as of the date of manufacture.
What the insurance calls it, what the title and tag department calls it, and what you call it when it comes time for repair can be different things. You will be servicing an IH chassis with the 7.3 engine (same thing as many school buses and IH trucks). As far as insurance, they'll figure it out and so will the title/tag office.