If you coming all the way to Denver, can I assume that you are not planning to tow your trailer over Trail Ridge Road in Rocky Mountain National Park? If leaving Denver on I-70 heading west, you will have 6% grades on entering the Front Range at Genesee, at Georgetown, on the descent from the Eisenhower Tunnel, and on Vail Pass, but all are either 4 or 6 lanes. If you are trying to just make time and not see many sights along the way, that is the fastest route.
If you are not in a big hurry, take I-25 to Colorado Springs, CO 115 to Canon City, and US 50 to Salida, Monarch Pass, and Black Canyon of the Gunnison National Park. From Montrose, take US 550 south to Ridgway, camp at Ridgway State Park (FHU or electric-only sites), then take a side-trip to Ouray and Silverton without the trailer. Next take CO 62 and CO 145 to Placerville (and maybe a second side-trip to Telluride), Norwood and Naturita, and CO 90/US 191 to Moab. Visit Arches NP and Dead Horse Point State Park, then get back on I-70/US 6/US 50 just east of Green River, and head to the US 50 exit at Salina.
Just a suggestion if you really want to see more of what Colorado has to offer than just Rocky Mountain National Park, Denver, and I-70.
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2015 RAM 2500 4x4 crewcab 6.7L CTD; 2016 Rockwood Signature UltraLite 5th wheel