Can anyone offer advice as to why my mpg has suddenly decreased from an average 18 mpg down to 13mpg? I have checked and had completed all of the obvious things like a tune up, air filter, O2 sensors, exhaust check, etc. It has about 129K on it.

Check/replace the Engine Coolant Temperature (ECT) Sensor and the MAF Sensor.

Tx Grease Monkey, I already checked the ECT, what is the "MAF" sensor?

Make sure the tires are at 35psi, and no brakes are causing a drag on a wheel.
The MAF (mass air flow) meter is on the intake hose leading up to the throttle body.
It looks like a jet engine when you pull it out. The vortex of air travels through it and no dirt or contaminates, should be on it.
Cleaned once in a while with electronics cleaner helps fine the tune.
This time of year, longer time needed to run rich fuel mixtures until engine warm up contributes to less mpg's. These 02's you mentioned. They're new or original?
You should be 18 or above.

Using full synthetic engine oil and transmission fluid will improve gas mileage in the winter.