They were doing OK, then the gov. started forcing states to run E10 year round which is not as efficient as "straight gas." Glad to see your mileage improve. I think people forget how massive these trucks are. Trucks are larger and much more capable than they were 10-15 years ago, today's 1/2 ton could probably out perform a 90's 3/4 ton. An ex. cab 4x4 also weighs in around 5,400lbs!!!
My wife's 2011 v6 mustang was only getting 19mpg when we first bought it. She drives close to a 70/30 city/hwy mix. It steadily increased and seems to have leveled off now that it has 3,000 miles on it. It now averages between 23-24mpg for the same weekly driving. We got almost 30mpg on a 300+ mile road trip
For comparison, my 00 5.3L A4 3.73 Z71 averages between 13.5-14mpg (almost 100% "City" driving.) If I tow my boat only around 11mpg. I
used to average over 15mpg until almost every station around here started running E10 all year. The stations that still have "pure gas" aren't worth the extra effort (20 mins from house) to get to.
Also, if you look closer under the big numbers on the window sticker, under say the "city mileage" it should say something like "Expected range for most driver's 12 to 18 mpg" Chevrolet doesn't rate their vehicles, the EPA does and you'll see the same thing on all cars.