1999 Lincoln Town Car POOR GAS FOR THE PAST FEW MONTHS

OK, IVE BEEN LOOKIN EVERYWHERE FOR MONTHS. I HAVE A 99 TOWN CAR, WHEN I BOUGHT IT WITH 22,000 MILE ON IT THE GAS MILAGE WAS GREAT, FOR A BIG CAR. I WAS GETTING 18.7 MPG CITY AND 24.8 HIGHWAY. SINCE THEN I AM NOW GETTING 14.6 MPG CITY, AND 19.7 HIGHWAY. UUURRRGGG. I HAVE THE TIRE PRESSURE CORRECT, NEW PLUGS, NEW AIR FILTER, NEW OIL, NEW TRANNY FLUID, ALWAYS CHANG THE FUEL FILTER EVERY 3 OIL CHANGES. I LIVE IN THE SAME CITY AND DRIVE THE SAME, IF NOT BETTER, TRING TO GET THE MPG UP, ALL HAS FAILED. ITS GETTING TO THE POINT WHERE I CANT AFFORD TO DRIVE IT. PLEASE HELP...