I have also had issues with my gas mileage over the years. I had a 2014 for 2 years and am currently about a year and a half into a 2015. Both totally stock with the 5.3L V-8.
I don't drive all that much (don't know if that has any impact, my 2015 only has 6100 miles on it) and both trucks, amazingly, have averaged 13.4 MPG, exactly. I'm not thrilled with that but I really only need to fill it up every 10 days or so, and gas is hovering around $2-2.15/gallon (although with the Giant discount tied to grocery purchases I usually pay about .75 cents a gallon every third fill) so it really isn't the end of the world but I sure seem to get worse mileage than most?
I'm in Philly BTW where it either seems to be 100 or 25 degrees.....