Fuel mileage dropped quickly from 21 or 22 mpg down to 16 or 17 mpg

If you have never replaced the oxygen sensors they could use changing especially with the mileage on your truck. Oxygen sensors give air/fuel ratio data to the truck's ECU which in turns regulates the amount of fuel the engine gets. So, the oxygen sensor, there is probably more than one, is the best sensor to change to improve gas mileage. The next thing to look at is your emissions system including your EGR valve, EVAP and even your PCV valve. To really get a good sense of what sensors and components are borderline, and I suggest this with the mileage of your truck, get the trouble codes pulled. Advance Auto or Auto Zone will do this for free.