We just got home from 63 days out west (Yellowstone and Dinosaur w/ grandson, then Yosemite, Kings Canyon, Sequoia, Death Valley, Saguaro...you get the idea).
For the same trailer; the Rockies require twice as much engine as anything back East in order to actually be comfortable on the passes. I wouldn't consider it without a diesel.
Another thing is that well maintained diesel trucks last forever. Plan now for what you might be doing in five years
As for the trailer; a wise saleslady (yes, there are such things) told us 12 years ago to consider our first RV a "training trailer". Buy used, figure out what you like and don't like, then look for something better. The Titanium is our third, and I believe our last fifth wheel. This one we love.