I just got back from a tow of a lifetime, which was just over 5700 miles in just under three weeks. I have to give a huge double thumbs up to the old 2003 Dodge Ram 2500, it performed just perfectly.
I averaged 14.1 mpg towing my 8100 pound trailer with probably another 1500
pounds of gear, luggage, dogs and people.
My route took me up and over the Rockies at the most difficult passage on I-70 headed west out of Denver.
My truck has the 5.9 H.O. Diesel, and it towed like a dream. Those miles long 7% grades are no joke, and I was climbing them with the cruise set on the normal 65. The downslopes were the same story, that truck would not be pushed around by the trailer. I did use the tow haul button on some occasions, but only for the longest and steepest sections. I know having the trans jump in and out of O/D is a rookie mistake that can shorten the life of your trans, and the descent is safer with O/D off.
When I tell people I towed my trailer from Chicago to California and back with a 16 yr. old truck with 290,000 miles on it, they say I'm crazy. I just tell them it's a diesel.