Sadly 21 is about what you should expect, o matter what they advertise.
Tricks for better mileage are:
Make sure tires are inflated to proper psi, higher pressures create less rolling resistance, but also promote wear in the center.
Remove all unneeded weight, heavy bumpers, tool boxs, etc.
Bug guards and window guards create ressistance, also bumper mounted lights and asstd items.
Tailgate open or closed you choose, to much for me to debate, I know what works for me and thats a bed cover.
Tune the engine and keep it well tuned.
low restriction air filter, synthetic lubes instead of dino oil or grease.
And maintain hwy speeds at or around 60mph
Surface driving should be done smooth and steady, act like theres an egg under the gas pedal.
I commute 70 miles a day in my Silverado and have it giving a steady 24 mpg by using these methods. My 99 Tacoma with a supercharger, 4.10 gears, and 35" tires gets 23 mpg if I follow these rules.