Poor Gas Mileage

I have a 1995 toyota camry le 2200 4 cyl automatic with 164875 miles the car has been running great in the last month or so my wife said that it had started to get poor gas mileage the plugs wires rotor and cap have been replaced 4 months ago and was running great getting 27-28 city 30 32 on the road now it is aboyt 17 it is now rough at Idle and about a week ago it started to smell gas real strong.I took it to the toyota shop and they cannot find anything wrong. Other than the piece of flex pipe under the enging was wearing thin that I was aware of and they said that that would not cause that problem, they say no codes are showing up and unfortunatly I dont think that they know where to look the car is still there. I also tryed to check them myself and got no where before I took it in. I know that randomly replaceing parts is not the answer and is costly. Is there some way that I can check the oxg. Sencor and others or do you have any idea where to look for the problem I an mew to this program of getting advice off the internet but hope it works out because I do most all of my mechanic work but am not real good a diagnostics and that is 90% of being a good mechanic If you can figure it out I will gladly pay to use this site thank you Dave