Yes the op did ask about real world experience from others about srw trucks in his original post . And a large percentage of the folks on here will likely never go up and down grades like we have here in the west .Anyone headed east from San Diego are going up hill for 60 miles before going down Mountain Springs Grade ,the same for folks leaving Phoenix heading north to Flagstaff ,so giving him a perspective about folks going to and from the dunes is about as good as it gets as far as truck capabilities . And yes Heartland does make smaller 5th wheels , just not in the Big Horn family of trailers.
As far as the 90 percent comment , it makes a lot more sense than buying a rig based on the fact that you may or may not go to the Rockies some day . Trust me I have been through Wolf Creek pass several times , and it is no joke , but anything that can run up and down some of these other hills will work equally as well there . Have a great evening