I just crossed 2000 miles. The mpg over that time recorded by trip meter "B" is 18.9 mpg. When not towing, I typically end each tank with an average in the range of 20-21 mpg on meter "A", which really impresses me. That is probably 70% highway mileage and 30% around town. The two tanks with extensive towing (100+ miles, tandem axle landscape trailer with mesh tailgate) ended in the 17-18 mpg range.
I see the engine in V4 mode very often, but was really surprised to see it in V4 mode when towing a full load of gravel a couple weeks ago. Once I got up to cruising speed and was motoring along on the flats, the engine hopped into V4 mode until I hit an incline. I guess once you're moving and up to speed, there isn't a whole lot other than aerodynamic drag and some frictional losses at play, so 4 cylinders is enough.
One big plus for towing is that I can now use the full capacity of my trailer; my previous tow vehicle was limited to 5000#. First time I picked up a load of gravel at the local stone yard with this new truck, it was really cool to see the scale heading up to 13,000# for the combined truck, trailer, and gravel load.
Got to try hill descent mode several times so far, traveling down a steep rutted road to my mother-in-law's river cabin, and it works awesome, especially in 4WD low. Coming back up the road in 4WD low is a piece of cake, slow and steady. I really like the electronic transfer case and controls, which operate seamlessly and won't let you accidentally do anything stupid.
I still haven't got a bed liner. The bed is developing a nice patina of scratches, which I like. This isn't going to be a Barbie truck! Though I did have a real mess a couple weeks ago -- I brought home some new crab pots, and left one in the bed overnight as my hands were full. Came out the next morning and it looked like a herd of raccoons crawled all over the truck. Not sure what was so interesting about an empty crab pot (and it was new so it didn't smell yet), but they got muddy clay footprints all over the truck in the process of climbing into the bed. When a downpour the next day had no effect on the clay, I grudgingly pulled out the bucket and hose.
If I could change anything about the truck, it would be to have firmer seats. My butt is calibrated to Japanese and German cars, and these GM seats feel like sitting on a loaf of bread. No complaints otherwise, which is remarkable for a nitpicker like me.