From my past experiences most problems with fuel mileage arise from small vacuum leaks. Any leak will create havoc with fuel metering-02 sensor will read that more air is in the system, and will cause the computer to richen the mixture. Now this can also happen with a lazy 02 sensor as well-they need to be replaced around every 100k miles.
I have had a thermistat go bad(stuck open) and caused my fuel economy go south -left the engine running rich because it wasn't telling the computer it was warm enough to go off the cold running cycle.
Of course you know about dirty air filters and fuel filters as well-fuel pressure regulators can bleed fuel past is't diafram and create turmoil all so.
Hope some of that helps
Poolman