bad fuel mileage on 2001 Dodge Grand Caravan

I knew what the codes where and have had it checked. at two shops the last one said bad oxygen sensor, but I have changed it twice already need more help

Asked by for the 2001 Dodge Grand Caravan
So you continue to have the P0172 and P0455 codes after replacing the oxygen sensor and erasing the codes? Does this car have more than one O2 sensor?
Are there any vacuum leaks?