FYI: I just pulled a bumper pull trailer weighing in about 11,800 lbs about 300 miles on hilly two lanes. I am very pleased how my truck pulled and handled the trailer. I had no problem maintaining the 65 mph speed limit.

 

more details please

Did you lock out 6 th gear?

Tow haul on?

Overview of the dowshifting? i.e. 5-4 on hills?

 

For about 20 miles there wasn't any wind and the road was fairly flat so the truck stayed pretty much in 6th gear, but when the hills started I locked out the 6th gear. Also every time I pull a sizable trailer I always use the tow/ haul option. I've read where tow/haul gives the transmission more hydraulic presser to help stop transmission slip. Plus tow/haul gives me downshifts when I touch the brakes. Lastly, the truck didn't downshift too much. There was some hills that was fairly steep with long grades which I used the (M)anual mode so I could keep the truck in 4th gear while putting the gas pedal on the floor without downshifting. If you have common sense and good comprehension skills, you can learn how to read the your truck and how it wants to pull.