My 2014 would easily do over 22 MPG if it wasn't loaded with tools and stuff in the bed and rear of the double cab. 5.3L, six speed and 3.42 gears.
It got really close to 22 the first trip to Tioga Co. in the fall of <span style="text-decoration: line-through">2015</span> 2014, when it was new and I hadn't yet got 'er where I wanted 'er to be - treasures-wise. Tools, log chains, air compressor, jacks, ratchet straps and all sorts of things add to the weight now.
But it also got hammered most of the way up and back on that first "almost empty" trip, often going with traffic in 65 MPH zones @ 75 or 80. When kept around 72 MPH on cruise running fairly level four lanes, it does well over 22 MPG - mile after mile.
Too many steep hills on that trip for best fuel mileage. Back when I had a 2010 Silverado that struggled to get 18 MPG going to camp, knew a guy with an identical 2010 who regularly went to the shore with his truck. Had no problem doing 20 or better going down there and back.