We're currently on a couple week tour of the National Parks in Southern Utah towing out 2011 27' FC that I'd guess weighs in at about 6800 #. Our TV is a 2001 Sub 1500
5.3 ltr with close to 130k on the odometer. Since buying our AS in Feb 2011, I've been looking for excuses to spend the big bucks for a "real TV" like a diesel truck. So far, the Sub keeps doing fine. This trip, we've crested three passes on Hwy 70 over 7k, the highest was just short of 7900 ft. Traveling east the grades seem a bit less steep than on the west bound side; I don't know. I'd guess they're in the 3 to 4% range. We crested the highest pass at 50 mph in third gear. I don't run the engine much over 3k rpm, so on the steeper grades, that means we come over at around 45. So far, with almost 1700 miles traveled we've been averaging just short of 11 mpg. We've been passed by more powerful RVs and more powerful TVs, but, heck, we're on vacation. So, all this is to say that I wouldn't rule out a gas engine TV.