The databases used on car parts websites and similar are assembled using data from several sources, of which DVLA is the least likely. I know this first hand as I had an issue with my Volvo several years ago. It was on many, but not all, databases as the wrong car, which caused a lot of issues every time I ran insurance quotes amongst other problems.
I have to give Saga credit, as the only insurer I contacted who showed any interest. They put me in touch with someone from one of two organisations that assemble the data which is then sold on to various users. He found that an incorrect data upload from the SMMT had skewed the details of my car, and managed to get it corrected.
But a common issue with coachbuilt motorhomes in particular, is that the bare chassis or back to back cabs as pictured by
@the green vanper above, are ordered and supplied direct from Fiat in Italy to the companies building the motorhomes. As such, they completely bypass the UK Fiat dealer network, so the information pertaining to them is not usually found in the dealer systems. They have no record of import, sale or customer details unless the vehicle is taken into the network for service or repair, and often their records are only then updated if a warranty claim has to go back to Italy, or the owner asks explicitly for it to be done.
But I always treat these databases with a large pinch of salt anyway. Again, for my Volvo, most car parts websites although now showing the correct car details, list oil and filters etc. for an engine that was no longer fitted to my car model at least three years before it was manufactured.