Horrible Gas Mileage

I have a 2000 Isuzu Rodeo LS, 2WD, Automatic transmission, 3.2L 6 cylinder with 129000 miles. My gas mileage per tank has dropped from 320 miles before the gas light would come on to 230 miles before the gas light comes on. Timing belt has been changed. Car has had tune up in the last three years, EGR valve, Idle air valve and water pump have all been replaced in the last three years. The mileage as slowly been dropping over the past four years. I have even used higher grade gasoline to on avail. I have had the injection system cleaned as well as having the spark plugs changed. The rodeo runs performs well. Acceleration is still smooth. It gets down the road with no problem. However, gas mileage sucks.