Your vehicle's computer will adjust for altitude as much as it can, as you go. I towed from the south Denver area to Breckenridge and back about a month ago with no problems. I had more issues on a long trip to the upper midwest last week.
Vapor lock does not exist on a FI vehicle. It is a PIA on my '57 Chevrolet in the Denver altitude with the heat we've been having.
I have traveled the million dollar highway for 30+ years. Beautiful part of the state. You may have your hands full pulling a rig like that, trying to enjoy the view at the same time. There are actually some campgrounds near the top, you may want to rest (and have a bump).