Half mpg?

Basic maintenance might improve gas mileage some, such as cleaning throttle body, filters and sensors, and a tune up, but you have a significant drop in MPG, you should have the timing checked as well as transmission, if you do a lot of highway driving the loss of OD can really eat up gas mileage. Something as minor as a tstat stuck open can reduce MPG as computer will remain in open loop mode longer (computer ignores most sensor inputs and runs engine at higher RPM longer). You may also want to verify that the CEL is actually working (should illuminate at startup), because things like an O2 sensor can also affect MPG, but that would also cause a CEL.

Fix the leak in the tank.