I am a flat lander, but I always tow to, and through the Canadian Rockies. I have also towed my RV to, and through Wyoming, Montana, Idaho, and Utah.
I am very familiar with mountain towing.
I am also a hiker, and mountain climber. I have an altimeter in my watch. The highest piece of paved roadway in Canada is the Highwood Pass at 7200'. The highest I have ever been with my RV in tow is just over 8000' in Wyoming IIRC.
So, I guess what I am saying is that until and unless I go to Colorado, I really don't care much how my truck would perform at 11,000'. It does just fine at 7 and ~8000'.