Gulfstream made up the name to describe a RV built on Class 5 Chevy Kodiak chassis, not necessarily diesel. Many Jayco and Gulfstream RVs on Kodiak were built with gas engines, as were some of Thor's Ford-based Funmovers.
Super C came to be applied to anything that size or larger: Ford F-series, small Internationals, Freightliners in Class 5 and Class 6 chassis, not just Kodiak/Topkick. The International came into the picture at one RV company when GM quit making larger trucks again, another moved to Freightliner.
You can find these built on larger trucks. Dynamax used Freightliners from Class 5 M series through Class 8 Columbia (as well as Ford E series and F series Class 4 and Class 5). Specialty companies build in sizes up through Class 8 on Volvo, Freightliner, Navistar, or any other big truck you want to buy that comes with a long chassis for a straight truck.
Depending on how big you are looking for (they range from under 30 feet and under 12,000 pounds to 40+ and weight ratings up to 40 tons) original price might have been about $80,000 or it might have been upwards of a million.