1998 Chevy Truck Really bad gas milage

I recently bought an old business owned truck that had a few problems that I have been trying to fix but I have noticed my fuel mileage has been dropping since I got it. It used to be near 15mpg, but the last I checked it got 8.6mpg. IAnd what really gets me is that the check engine light hasn't come on. It my be a little off because its winter and I idel more but it shouldn, t be that bad. I have new spark plugs and wires, new air filter, new fuel filter, changed the oil recently. I think it has most of the factor parts on it to. My guess is a sensor/s. And one more thing one of my catlytic converters is no more. Its not cloged its completely empty but I heard that O2 sensors only can change mpg by 15%. I have no way to test them either and I realy don't have money to have a dealer look at it.