Yellowstone was a Gulfstream brand, used for rental market, in that era. Sort of a Gulfstream equivalent to Winnebago's Chalet brand or Coachmen's Pathfinder. Rental market build quality was usually the same as retail lines, but furnishing chosen to be harder to damage, easier to clean, at the expense of glamour.
Later in Gulfstream's history, before they stopped making motorhomes, I think just before maling them all come from a Conquest "division" and changing to that as a make, Gulfstream used the Yellowstone brand as a retail brand, as it had been in a more distant past, both motorized and towable.
Gulfstream has since gotten out of the class C business, not to say they couldn't get back in if the investors so decided.
Don't try to read too much into RV brands. Marketing departments at most manufacturers are constantly changing how they position these names, just as do auto manufacturers. Top of the line like Belair or Fairlane can become entry level in a few years, or the name of a whole new model line years later (Malibu, Nova, Galaxy, Impala, Ranger all started out as trim levels rather than model lines).
