Welcome, BlockParty! I know what you're talking about regarding tow vehicles and the Rockies. When we got our 17ft HiLo, I had a Nissan Frontier (6 cyl). It pulled the HiLo, but when we took it to Colorado and went over Monarch and Red Mountain Passes, it was doing it in first gear and at very near max rpm just to maintain about 30 mph. Then, we had to navigate a long, 7% uphill grade out of Camp Verde in Arizona (again in 1st gear) with the temperatures in the 100s and things started getting hot. I decided I needed a new tow vehicle.
I got a 2005 (used) F150 and it was able to pull the trailer with little effort. We've been to Rocky Mountain NP twice with the newer combination and I've even pulled the trailer up a long 13% grade to Cedar Breaks in Utah with no complaints (but it had to be in 1st gear to do that too).
I really recommend you shop around for a recent year used truck in the 150-1500 category. They are out there, and will make towing so much nicer. Besides, there are too many beautiful places in Colorado that are just waiting for you!
Hi-Lo 1707T - Tire Minder TPMS on Tow Vehicle and Trailer, 300W Solar Battery Charger, Equal-i-zer WDH
2014 F150 Platinum 4x4 3.5L EcoBoost SCrew