1997 Chrysler Town and Country Mileage loss after O2 swnsoe

Engine Performance problem
1997 Chrysler Town and Country 6 cyl Front Wheel Drive Automatic LX miles

My car ran just fine when the check engine light came on. The problem was diagnosed by the service center as a bad O2 sensor (rear). The sensor was replaced. At the same time I had the oil changed and the fuel system flushed. Following the repair, the car runs just fine. Easy start, good power, smooth running, etc. The problem is that my gas mileage fell off by about 2- 3 miles per gallon. I took the car back, the problem was not found so they replaced the O2 sensor again and I have the plugs and wires replaced. No improvement.
Is it possible that the front sensor needs to be replaced too? I don't have a check engine light problem. Or, does the sensor need to be recalibrated somehow? I read somewhere that by disconnecting the battery for a period of time, upon restart, the sensors will recalibrate for fuel flow. Is that true. In any case, the car runs great but the mileage is bad.