For the rig in your signature a newer 3/4 ton gasser will do fine, even in the mountains. We use a 3/4 ton Chevy to haul our 32' TT around the four corner states always traveling at elevation and we always get to our destination without issue. Just rev her up to her power band and we are good to go.
Our truck is dedicated to towing our rig and just sits in the garage awaiting duty, total waste of money for us to get a diesel as we would never realize any savings. As long as our TV has gas we know it will get us where we need to go, paid off and never has needed any service other than oil changes and filters.