We just moved out to Colorado and had our first chance to take the motorhome through the Eisenhower tunnel. At 11,000ft the v10 really runs out of air and it could only maintain around 35mph on the long 6% grades. Tranny and water temps never budged from normal. Staying in the right lane you will often find yourself behind a slower moving semi-truck with not enough power to pass. Downhill in 3rd or 2nd gear made it easy to stay off the brakes. Adhere to the speed warnings posted for big trucks and you'll have no problems.
Taking your time and enjoying the scenery is great advice.
My neighbor at the Tiger Run RV resort in Breckenridge had a 625hp Newell. He said he could hold 52mph on the same section but his 400hp Country Coach only did 35mph.
2009 Georgetown 350TS (bunks)
400W solar, 440AH 6V GC2
2009 Ford Edge AWD Ltd towed
2011 Honda Fit Sport towed