Ok, this is only a 30 mile review but I was able to get a feel for towing a tall 5400lb camper. I'm going on a bigger trip next month with a real test grade, I'll report back on that.
First impression, it comes out of the hole as good as my full size did, the power was surprising. Of course I'm not drag racing here, I've learned that a little patience when towing goes a long ways. The collie was also more stable and settled feeling than my full size is. (It's an 04 with188,000 miles on it, so there's that.)
Tow haul mode seemed a little busy to me on the rolling hills, although I'm not a fan of manual mode, I opted to use it. More miles on a different route will probably change my mind on tow mode.
A rant on "manual" mode, it should be called extra automatic mode. It downshifts (stupid) it holds shifts forever after up shifting unless you let off the pedal(more stupid) My wife's Malibu is 10 times more manual than this thing is. I know how to drive a truck on hills and don't need a foolish computer helping me. /rant
It did better than expected on the hills, pulled the biggest one at 55mph 3500rpm in 3rd I think. For the record 4K doesn't feel as wound out as 3k on a V-8, so even though it sounds high, it doesn't feel that way. I got 13 mpg according to the dic, which has been spot on with the previous full tanks I have hand calculated. That's better than my V8 and would surely do better on flat ground.
The bottom line is that I was pleasantly surprised and overall impressed with the collies performance. I expected to be pushed all over the road, and wasn't. I wouldn't recommend going any heavier with an RV, but would have no problem going to the max with a flat trailer.
I do have a lot of towing experience over the years, so don't go out and buy a giant RV on my recommendation.
TL;DR In capable hands this is a very capable truck.