Some DMV offices seldom register an incomplete vehicle. As long as it's not registered as a Prevost, they just need to lookup incomplete vehicle process. If the look at the factory identi-plate it clearly states incomplete vehicle.
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I do think it's strange the final manufacturer doesn't have to assign their own VIN like many rolling chassis motorhomes. They don't have to though.

DO NOT let them register it as a bus. You then become subject to the requirements of commercial operators. You will also have a vehicle that's older than it really is.