I found the answer! (at least in CA) This is a quote of the DMV pamphlet, General Registration Information, section 1.105 (CVC 671):
Second Stage Manufactured Vehicles
When a vehicle is completed by adding a structure/equipment (second stage) to an incomplete vehicle (first stage), the VIN of the first stage is used for registration purposes; however, the year model, make, and body type are determined by the second stage.
Example: A 2009 Ford chassis has a 2010 Fleetwood motorhome body attached. The registration would correctly show 2010 Fleetwood motorhome with the 2009 Ford VIN.
Thanks for trying to help, it encouraged me to look farther.