Never hit a mountain highway in the Western mountains where I could not keep up with traffic, and passed quite a few trucks in my GM 8.1L powered Monaco coach, with the Allison transmission, while towing a Chevy HHR. I do have the UltraTune from Brazels, which helps but I really didn't have any serious problems before I got the specific tune for mountain climbing. I never saw anyone in any RV at single digits altho I guess it is possible if they were having overheating issues which can happen in any coach, diesel or gas.