Interesting question! From what I read, I would think it is likely a 2019, though it could be a late 2018. RV model years usually change in the spring; April/May.
It is not unusual for the engine and chassis to be a model year earlier than the completed coach as the supply chain can be a drawn out process. Officially, I think you probably have a a 2019 model-year RV but the 2018 VIN certainly provides a point of contention. It definitely isn’t a 2020.
Good advice above on using state resources and/or legal representation to get the RV returned and the full purchase price refunded. The first step is taking it back to the dealer and asking (in writing) for a full refund. If there is any hesitancy, contact the consumer fraud agency in the state where the RV was purchased. And, get the attorney to write a letter to the dealership stating your case in legal terms and asking for a full refund plus attorney fees.
If that doesn’t work, you’ll need to let the consumer fraud complaint and legal action run its course. I would park the RV and not use it at all to minimize the chance of a counter-claim that you have caused damage to the unit.
TJ