First the 18 to 21 you see 'everyone getting' is not the norm so question the sources.
What you are reporting is more the norm so you have nothing that is a big issue.

My 172,000 mile perfect running 4.6 is at 16 to 17 on the best runs with 14 to 15 short run averages.
Some are going to dissagree but you have to live in reality.
Your running about 4800 to 5000 lbs in weight +/-.
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Things to check out that may improve the mileage are new OX sensors, check the fuel pressure is not to high, Thermostat is not running the motor to cold.
Run tires on the high side of pressure and be lite foot consious while driving..
Don't even think about super air intakes or filters. They won't do anything for you.
Good luck.