I could not tell which route you will take. This is my experience....
I towed from Denver to Utah on I-70...over Eisenhower Tunnel and Vail pass. Both are over 11,000 high. See my sig for the rig I towed. The only hairy part was coming down. It pulled uphill fine, albeit in 3rd gear. I just took it out of ovedrive and kept it out.
When it comes time to come down from the pass, I would slow to 25 MPH or so, put it in 2nd and creep down the other side. One and off the brakes to keep them from heating up. I passed a semi flatbed on the donwside of Eisenhower with smoke billowing from the rear brakes. He was carrying a pre-cast concrete beam for a bridge and rode the brakes a bit too much.
Now...a few things.
1. Stay the hell away from Monarch pass towing your rig. I towed a pop-up with a Ford E-150 up there and was just glad to get off that mountain. Just my feelings, but good counsel, I believe.
2. Don't tow on the road north from Durango. Drive up there if you want, but stay off with a trailer. If you must tow over it, leave you name, address, and next of kin with the Sheriff's office in Durango....they will have a good chance of needing the info.
3. If you're really going to run all over Colorado, buy a book that lists all the passes and describes the grade, length, and special problems of each pass. (For Example, 6-8% downhill for 3 miles with a STOP sign at the bottom.) It's worth it if you want to nose around the state.
4. You can't know how the road will be from the map. The road from I-25 to Great Sand Dunes NP looks, on the map, like it would be a bugger. But it's so gradual, no sweat.
If you'd like to ask about other roads, I'd be happy to tell you what I know.
(now in Whitehorse, Yukon Territory)