Frustrated With My Voyager

My fiance' and I just bought a Chrysler Voyager (2002) about 2 months ago. About a month ago the service engine light came on. We took it to the local Auto Zone and they hooked it up to their computer for us. I can't remember exactly what the code was, but they said it was probably just the gas cap so we replaced it to no avail. We went back and they said usually the only other thing it could be would be a vacuum is loose.

The question I have is would a vacuum being loose cause the fuel mileage to be horrible? The fuel mileage has been fine until about 3 weeks ago, then all of a sudden it got horrible.

My ex husband and I are still friends and he said it could be one of two things. He said it could be the additives they put in gas in the winter (we live in good old PA where it is freezing) or that a sensor may have gone bad. They just did a tune up at the dealership before we bought the van so I am not thinking that is the problem.

What do you all think? My ex said he is thinking the PCM or OEM sensor.

Thanks a bunch!