Believe me, some of the lower powered diesel coaches will also struggle on the long high passes, but they will get you to the top and back down. I have had both diesel and gas coaches and have never heard of any coach NOT making to any destination because there was not enough power.
There is a big difference between being able to get to the top and getting there comfortably and gracefully without blocking traffic on the way. We have always had diesel, and a moderately high-powered one, because we prefer to motor along without concerns about either making it or holding up other people.
We had no trouble finding a DP floor plan that works for us, and we get 10 mpg without our toad, 9 with it.