1999 Honda Accord

My Honda accord 1999 has been getting horrible gas mileage I have owned it for 7 yrs. And have never seen such loss of gas. The car runs fine and no stalling or sputtering is occurring.I do smell gas from time to time lately.I have searched for leaks often and even had a mechanic take the gas tank out and search again after 6 hrs of work we found nothing. Gas cap area smells like gas a lot when I stop and smell the area. Heard about o2 sensors but Honda wants so much cash to test them. My question is can I test the sensors myself or am I at Honda mercy?Is the other things that I can test myself. My tires are fine and inflated properly.I have replaced my flex pipe connected to exhaust as well. Can be as simple as the gas cap?There is no filter but in the tank itself. Is there something in there I can replace? Thank you very much for responding!