Short answer is yes, a model year 2024 motorhome can have an earlier year chassis. An internet search on the subject will yield a lot on that subject.
Specific to this topic, federal law allows a vehicle manufacturer to identify a vehicle by a specific model year so long as it is built after January 1 of the model year change-over. For example, my Freightliner M2 was built in February, 2023, and identified as a 2024. Conversely, I have seen Ford trucks built in late November 2019, identified as a 2019, well after the traditional late summer change-over.
Brian Clemons has mentioned that last year was a bit of a grab bag as Freightliner transitioned to the M2 Plus, as well as DD8 to L9 change. If you get both chassis VIN and upfitter (Forest River) VIN and utilize the NHTSA VIN decoder, it should answer your question on that issue.