Changes of title from bus to motorhome must be recorded by the DMV authorities - I don't know about in the States but in Europe there's a Public Information Request process whereby anyone can apply to receive statistics and data of that sortJeremy
It might not be that hard to get a reasonable idea from existing title/registration data. If you can get a list of RV by manufacturer, you'll get close pretty fast. Of course this is by state but an MCI registered as an RV is a strong indicator it was converted at some point. Conversion only might also show up like Prevost RVs.
Some vehicles, such as those used exclusively for farm use or on private roads, will never be registered. I've also encountered some governmental agencies that will operate vehicles for years without an official title or registration - an example is a '52 Suburban I purchased from the Illinois forestry department years ago which they had received from the National Guard. The vehicle had not been titled or registered for many of the years it was operated in the State Forest - apparently a paperwork oversight. Was a mess when I went to get title and registration in my name.