Any coach you pick, whether gas or diesel, will make it to the top of any Western mountains. Unless you want to spend really big bucks, the speed while climbing that mountain is all about the same, in spite of what some DP owners will tell you. If you know how to drive, descending that mountain is also no problem as thousands of people do it every year with both gas and diesel coaches. Find the best floor plan that suits your needs.
Now, for off roading you need to consider what emergency road service will cover. CoachNet will tow you, if stuck, on any dirt road while going to or while in any designated RV park, whether it is private, state, or federal run.
If you want to travel down some undesignated dirt road, and you get stuck, CoachNet puts a limit of 100 feet from the end of a paved designated road. This means that if you want to head out on your own, and you get stuck, you may also be stuck with a HUGE towing bill.