2017 Class A's

The model year is more a designation of the design and design updates, rather than a precise indication of when a vehicle is built. A maker is of course free to introduce an updated design at any point during the year, and often change the model year designation at that time. The only requirement is that vehicles made on January 1st of a year be in that year's model year, but it can start any time prior to that (within the previous year) and end any time after that (up until the end of the year).

If a maker shuts down for a few days around New Year's, it's entirely possible to skip a model year without having any significant break in production.

This isn't unique to RVs; it applies to all motor vehicles.