Did you get new tires recently? Are you sure you were getting mid-20's before the "drop"? According to the EPA, a 2002 4 cylinder automatic Camry was rated 21/29. Mid-20's seems reasonable for mixed driving. Any chance you have a brake dragging? After a long drive, feel the wheel hubs (be careful) - if one is especially hot, suspect a dragging brake. Any chance they changed the gas formulation in your area (where are you)? Any changes in your driving pattern? Notice any changes in the transmission behavior? Any chance you have a teenager? Or maybe you park your car in a vulnerable area where someone can steal gas? How good are your gas mileage records? Basing fuel mileage on a single tank average can be very misleading. You need to compute an average over 3 or more tank fulls. If your check engine light is not on, it is unlikely the problem is your engine. Do you smell "gas" when you first get out of your car or when you are driving with the windows down? A leaking connection could explain everything. Are you in a rust prone area? I've heard stories of Camry tanks rusting out on top...could let some gas slosh out, but you should smell this.
Waste of time and money. The sensor behind the catalyst is there to verify the catalytic converter is working. It is not used to adjust the fuel mixture.
Well if nothing else comes up, I'd suspect a plugged exhaust system. The dealer should be able to measure exhaust back pressure.
Here are my thoughts about the cause - most likely to least likely -
1) Poor mileage computation 2) New tires 3) dragging brake(s) 4) plugged exhaust 5) changed driving habits 6) changed gas formulation 7) slipping transmission 8) engine mechanical problem 9) fuel system leak (you should smell this) 10) someone is stealing your gasEd