There's nothing misleading about it. I understand your reasoning, but this is not the world of cars/trucks; You are not buying/selling/registering a bare chassis, you are buying/selling/registering a complete RV, and while a major chassis change may very well make a difference, I would suggest no one would actually consider a lesser alternator as "waste of time" on a used RV, as you suggest. The manufacturer's brochure will normally list the basic drivetrain mechanicals that apply to the first units built in that model year, and then there may in fact be upgrades midyear (when the newer chassis begin their trip down the coach assembly line). So you are actually more likely to get better than the brochure lists, but never worse in the case of split year RVs. Doubtful that you would complain about those improvements/upgrades, right?
I live in CA. My last coach was a 1988 Jamboree, built in 1987 on a 1987 Ford Chassis, and that chassis was actually built in late 1986. So was it a 1986, 1987, or 1988? It was correctly registered as a 1988. CA does in fact, follow the Federal guidelines (I'm sure that exceptions occur, but they are the exception, not the rule).
The only hassle is that when purchasing chassis or tune-up parts, you will need to purchase for an earlier model year, particularly if there were changes. I had to remember to buy for a 1987 rather than 1988, as there were significant changes at that time (Carb vs FI, for example). That was indeed a big change, but I bought the rig knowing it was carb.
Keep in mind, if the chassis and coach had to match, there wouldn't be any current model year coaches even available until mid-year:(. You want to purchase a new 2021 RV? You would need to wait till June of 2021.
