I think you would have to use the VIN number that came with the Chassis. That is to say, whatever VIN was provided by Ford or by Workhorse or by Freightliner, etc. I have a dodge based winnebago, for which chassis there is a dodge VIN decoder that somebody posted at mytravco.com. That only only works for the RVs made up through about 1978 on dodge though.
For anything made after the early 80s I think the VINs were standardized, as per federal regulations.
VIN reports for RVs may or may not return any info, depending upon the state that the RV is registered. Apparently, there is no federal requirement that sales be recorded. For the two used MHs we've purchased, we got a complete record in one case, and nothing in the other.
Our experience using carfax as part of our rv research was more than worth the cost.
It may not deliver the same info as a car title search but it will show each title "event" and the usual wreck or other significant things a buyer would want to know about.
It will show mileage at time of each title event and you still get a good idea of how many times class A bought/sold and when and where and mileage at the time of event.
One example: We were very interested in buying a pre-owned HRambler Imperial from a nearby dealer til carfax showed a "no true mileage" and it looked like no mileage recorded on odometer during early life of vehicle.
Dealers are legally obligated to disclose little things like that (they didn't)in state we live in and we walked away from that deal and glad we ran the VIN. Report on the one we did buy assured us that dealer representation accurate and coach mileage reliable.
We recommend carfax as a worthwhile part of Class A buyer research.