snow0700,
Nothing is wrong. Many motor homes are just like yours.
Motor home production years are introduced earlier in the previous year than the chassis manufacture. Therefore many motor homes are built on a chassis that is one model year older. Actually it is not unheard of that the chassis model year is 2 years older than the model year of the motor home. The RV manufacture may have had a lesser than expected demand, and was not able to use up all his Ford or Chevy chassis that he had purchased 1.5 years prior.
Our first motor home was like yours with mismatched model years. I admit I did not like it, feeling I got a one year old rig. But that is how it goes for many people who don't realize that and fail to ask the right questions. The good news is that your 2018 Chevy chassis warranty starts the day you bought your motor home.