if all else is equal, and you can afford the initial purchased, I prefer diesel for these reasons:
1 - fueling at the truck pumps makes life so much easier when traveling in an unfamiliar area which we normally are. Trying to decide if we will fit in a "regular gas station" can be difficult. If we can get in, can we get out again?
2 - normally better braking setup
3 - generator is normally away from the bedroom sleeping area (quieter while parked)
4 - engine is normally away from where the driver and passenger(s) are riding (quieter while driving)
5 - typically more cargo carrying capacity.
That being said, you can absolutely travel in the mountains with a gas coach. We did it for 5 years in a 34 foot motor home with one slide. Our longest trip at that time was 3 months and we still had a sticks and bricks house to keep all our "stuff" in.
We learned a lot about rving and ourselves as rvers during the time we had the gas coach and we don't regret having it. We did know that we definitely wanted a diesel coach once we decide to sell the house and go full time. There was no way, we could safely travel in the gas coach with the amount of "stuff" we wanted to carry with us.