I've got a 2008 5.4/3.73 CC 4WD with the TS auto..
I don't tow quite that much, usually around 6k-6.5k bumper pull enclosed trailer (some aero drag as well). Mine is not broken in good yet (only 5k miles), but averages: 9-10 mpg towing 6.5K enclosed trailer; with family/gear/dirt bikes light load 15-16 mpg highway; 12-13 mpg in town. If you can find a 4.10/4.30 it should give a little extra grunt with little/no fuel penalty. I think the 4.30 has a larger ring gear, but it's all overkill anyway.
I'm happy with the power, but it does rev/downshift a bit on steep grades. It maintains speed pretty well though, it's not like a '60 VW bus up a mountain! The TS tranny does a great job of putting the engine in the proper rpm for the condition at hand. I'm usually a manual transmission guy, but I found a great deal on my leftover '08 this spring and got the auto - it's much better than I expected.
The chassis is an absolute tank - I think it's basically a 1 ton. Chevy and Dodge don't really compare to the overall chassis package. It's quieter on the road than both of my cars, a bit more bouncy of course. I love this truck and glad I didn't settle for a half ton.
I tow a fifth wheel that is 8k dry and 9.2k to 9.5k loaded to camp. We have towed and camped 4 times since buying the camper. The combo does make you wish for more power in the hills, but on fairly flat ground it tows mine great. The engine really has to rev to maintain 65mph on a long incline, but this engine is made to do just that. I average around 7 to 7.5 mpg towing my 5'er and 14.7 to 15 on my daily commute to work.
One thing I discovered is that if I leave my bed cover in place when I tow, my mileage does drop to 7-8 MPG and the trailer feels 5000 pounds heavier and nervous. If I remove the bed cover, the trailer feels lighter (even though it isn't) and my mileage comes up to around 10 MPG.
Like I said, aerodynamics has a lot to do with it.
On conventional towing my 2010 F350 5.4 4.10 pulls close to 10,200 pounds up the mountains twice a year 60 hp tractor loader backhoe and grapple on trailer to my cousins house in speculator 30 miles of the trip are up a steep grade the engine revs high but it maintains a 65 to 70 no problems at all mileages around 10 miles a gallon until that steep grade then it's probably about 6, but what would you expect I came from an F150 5.4 4.10 which used to pull the same trailer and it sure scared the hell out of me going up and coming down that hill would push you right off the road or so it felt used to get anxiety thinking about the trip now it's fun :-) don't even know it's back there oh by the way I didn't have the backhoe unit on during the F150 trips which saved about 2000 pounds. I've owned a few different brand trucks with 3.73's I can't imagine it would be that much of a difference, the super duties are perfect at towing due to their size and steering components, brake systems, transmission is incredible I've had Chevys and dodges transmissions no comparison, the Ford blows them away I agree with everyone else the transmission just finds the perfect spot and never lets you down you won't regret it take care
Wow that kind of mileage pulling a trailer. I just bought an 05 CC S/D 5.4, auto, 4.10's lift kit w/ 35's and got 11.1 empty going from Kenai Peninsula to Anchorage some flat driving and some mountains and driving sensibly!