I repeat because it bears repeating.
Having a 1989 Southwind MOST LIKELY means it is on a 1988 chassis.
It is common for Motor home builders to buy a lot of chassis' ahead of production. IN order to ensure they have them for the start of their model year.
Fleetwood WILL HELP YOU determine the year of the chassis.
The 1989 year is the year assigned by Fleetwood to the completed unit.