Every PC has two VIN numbers, a chassis VIN and a PC-fabricated VIN. Like Taildragger states, the PC VIN will be a sticker located on the driver's door frame.
The chassis VIN number is primarily used for title, registration, and insurance.
I suspect the PC VIN number is utilized by the factory for record keeping, a unique ID to look up on their database to determine how they originally built the motorhome, the specs, features, options, colors, fabrics, and more. So if you are not the original owner of your PC, have both VINs handy when talking with the factory for refined help with replacement parts and such.