Towing a nominal 10000 lb 5th wheel trailer is a breeze - the truck is working hard but it is entirely capable to do so. The only problem after 20000 miles is that the rear disc brakes failed (bad rotors), but the dealer made good on the repairs.
5 liter engine combined with the 4.15 axle and HD substructure makes this an excellent trailer towing truck, in a standard body size.
Improve the day-to-day gas mileage above 12 mpg. Sure, 9 to 9 mpg when towing a 10000 lb trailer is normal, but do something for day-to-day usage.