I bought a '98 a few months ago and have been running it w/o any problems. The other night bought gas at the same place I always buy it, and yes I buy the cheapest, and within 5 minutes had rough idle, loss of power and have used 1/2 of the tank within 3 days having gone about 20 miles. I thought at first fuel injector cleaner, bought some, followed directions, put in tank. Not much of a change. My son bought some dry gas and put it in, still not much of a change. So now I am thinking fuel filter, but where is it ?? Owners manual doesn't seem to tell. The former owner had all paperwork, but never could see where the filter might have been changed. This vehicle has 133k on it. Any help would be appreciated.

Doubt it is the filter, FWIW it is on the passenger side of the firewall, somewhat low lying.