Maybe because people will no longer buy a Class A that does not have slideout rooms? Lack of slideouts was what made it possible for Fleetwood to build a full basement Stormy (or near equivalent Flair) on 12,300 pound G-30 or 14,000-16,000 pound P-series chassis.
I don't understand the "true motorhome chassis" part of this, since your example Stormy was probably built on the step van P-chassis, but at that length might have been built on the G-30 stripped chassis borrowed from the Chevy van. The E-350 and E-450 stripped chassis used for smaller motorhomes today are the modern equivalent of the chassis Fleetwood put under the Southwind Storm and the Flair. I went through many of these mid-90s Fleetwoods when shopping in 2004-5, paid careful attention to chassis being used, and was finding the G chassis on these lower range Fleetwoods up to 30 feet.
Technically, the stripped chassis of the Sprinter and the modern E-series are well advanced from Chevy G and P series. Better geared, stronger transmissions compared to the old TH-400, overhead cam engines with multiport fuel injection, larger brakes with vented discs all around, ABS, springs that don't need booster airbags. Superiority of the chassis was part of what directed me away from these earlier A gassers. We would not see better chassis under low end A gassers until Ford upgraded the F chassis in the late 1990s, and Workhorse created a lighter W-series line to replace the P-series. But even these were shared between motorhome builders and van builders.
I also doubt that this model is 25 foot long, it is probably over 26. The Stormy "30" in our RV club is slightly over 32 foot long, that's just what Fleetwood was doing with model names then.
