So I have towed my 10,500 lb 5vr with both gas and diesl almost the exact same truck otherwise.
2009 GMC 2500 6.0L gas, WT, tow package, 6 speed tanny, tow/haul mode, 3000 mile round trip from Kansas to California coast. 6-9 mpg avg. I never could use cruise control, it was a painful experience towing with your right foot for 3000 miles. Truck was constanty down shifting to compensant for lack of power. Had to run in manual shift mode no higher than 5th gear. Coming back east over the continental divide on I40 I pulled about a three mile 6% grade and was turning 5000 rpm and could only get about 40 mph by the time I reached the top. I think that trip took 20,000 miles off the life expectancy of that truk.
2013 GMC 2500 6.6L diesel, WT, tow package, 6 speed Allison tranny, towhaul mode, 2000 mile round trip from Kansas to Pheonix. 10-12 mpg avg. Used the cruise control almost the entire time except for the steepest/longest grades between Payson, AZ and Pheonix. A much more pleasant and comfortable trip overall. From Kansas to Holbrook, AZ tranny down shifted once. I don think I ever turned more that 3500 rpm on any 6% grade.