Yellowstone with an older Tow Vehicle...Would you?

2006 is not old, we did 1,100+ miles in 100*+ with the AC on, at just under 17,000lbs...it's a 1990 F350 gasser, averaged 7.5 mpg at 62 mph for the most part. Some climbs had me down to 38 mph in third going into a 20 mph head wind. Went through 14 gallons in 68 miles....that was a long climb.