Yes, the year difference on motorhomes is very common.
Let's say the chassis is built by Freightliner in Sept, Oct, Nov or Dec of 2012, but the MH maker Fleetwood doesn't get the body built on it until maybe Jan, Feb, March or April of 2013 and they built it to the 2013 model design - they're going to call it a 2013 vs the chassis mfg having built the chassis the previous year. That's the way it is and nothing anyone can do about it.
It is possible for the chassis builder to make the chassis early in a year, deliver it to MH maker who turns out the finished MH in the same year.
Bill