I have a cabin in Colorado at 8800 ft. For me to go to town I have to climb 200 ft to the top of the hill in about 3/4 mile and then descend down the canyon 4000 ft to town. In both my Suburban or Jeep I see very little difference below 7000 ft. Above 7000 ft performance trails off. Because the speed limit is 45 mph I don't really feel any lack of power. But if I were trying to go I-70 at 65 mph to the top it would be a different story. IMHO it depends how high you tow and at what speed that will make a difference of how a person perceives towing at altitude. Going 45 mph on a 70 mph freeway seems slow. Going 45 mph on a 45 mph road seems ok. Same 9000 ft altitude.