I agree with the diesel prognosis in the mountainous parts of the country!
While driving along in Utah, Idaho, Colorado, Montana and Wyoming a long with other RV traffic, you can spot the biggest difference for me as soon as you hit some of the 3 or more mile hills, the gas RV's begin to slow, and all the diesel rigs move out and overtake.
Coming up from Southern Utah in the company of a couple or three RV's, two Gas class C's and a Gas Fred A, as we hit the mountains South of Cedar City, I was held in traffic as they started to slow, once the next lane was clear, I was able to move out and roll past them on the up grade... by the time we crested out they were not in sight behind us. In a 20 minute stop at Cove Fort, they caught up, then passed us, and we soon passed them all again North of the fuel stop.
I don't think they stopped anywhere on that bottom end of the 300 miles from St George to Salt Lake City, but I gained a significant lead on the longer hills. Cruiseability means a less stressful drive, and a less stressful drive means an easier day, and an easier day for me means a less stressful day for the family, etc!