We do most of our travel in the Western mountains and I have passed many DP and other gas coaches on the climbs, with my Workhorse gasser. The Ford gas coaches have about the same horsepower and torque as I have so a lot of the climbing difference is the same as what I see on the highway. Many RV owners drive their coach at a much slower speed than what it is capable of, in the mountains and on the flat lands.
I have never heard of any of the Ford chassis motor homes that could not get to the top of any mountain pass. I am also sure that any gas coach out there can drive in excess of 75 MPH which is the rated speed limit of most RV tires. On our current coach I am now pushing 85,000 miles and even tho we travel in the mountains, I would guess that less than 1% of the time are we actually in a climb so steep that the transmission has to double downshift to make it to the top. In fact, it happens so seldom that to me it is a non-issue.
Going to Yellowstone we have never experienced any problems at all. There are certainly steeper stretches of highways in the Western mountains than the various approaches to Yellowstone. Get the floor plan you like since 90% of the time you will be parked and relaxing.