Welcome to the forum.
Yes they date the Unit by the body not the chassis. My case in point I have a 2016 Vegas 24.1 on a 2015 Chassis, an E350 with the 5 speed transmission.
Yes some of the newer 2016 Bodies have a 2016 E450 with the 6 speed transmission. If you go to the thread "Cannot shift out of drive" you will see some of the speculation for the change. There are others much more informed on this Forum that can go into greater detail.
Sheree & Michael NCNG LTC(ret) with Munpi and Sheba (cat).
2016 Vegas 24.1 E350 5 speed "Ms. Enterprise", Bug-Guard Screen, Recliners, Infotainment Center, HUD, ARC Shower Rod, Safe-T-Plus, Curt SpareTire Mount,Tornado Black Tank Rinser, ext. WiFi Antenna, Roadmaster Tow Sys& Invisibrake.