Need feedback from owners of Silverado PU's, 2011 and up.

I had a 2012 1/2 4x4 with the 5.3 and tow package. That truck was the biggest Pos I ever owned. I got a best of 18.8 mpg on the highway. But most of the time is was between 16-17 and only got 11/12 on E85. The truck couldn’t pull a hill going 75, hit any hill and it would down shift., towing a small car felt like you was towing 10,000 pounds. The biggest gripe I had was my truck had a miss at idle and after taking it back to the shop to look at, GM said it was normal. I bought the truck used with 32,000 miles.