When I was a member of the Explorer forums, some guys would "refresh" their O2 sensors by removing the sensor and burning the carbon and other deposits off with a torch. Apparently these sensors are plenty strong to take the heat.
My 2014 struggles to get anything over 13 MPG city/highway (mostly highway). This is with new plugs, new air filter, MAF clean, throttle body removal + complete clean, new serpentine belt and idler pulleys, new water pump, new coolant, synthetic motor oil and all new fluids in diffs, tranny + transfer case. Fuel economy is still garbage, even though I drive it conservatively.
I might actually try putting in new O2 sensors in it this summer to see if it helps. I've read that the injectors on these might also cause piss poor fuel economy as they age, even though they still work. It also has a passenger-side phaser tick when warm and I've read that fuel economy issues can be caused by timing issues as well, which the 5.4L 3v is famous for.
So, all in all, it could be a hundred different things, all of which will cost you and are a crapshoot. Isn't all this new "fuel saving technology" just awesome, lol?