If you do find that the mileage is low, ask around about a shop that does good diagnostic work and pay the $80 or whatever to have them at least do an engine report for you. Their knowledge and advice is sometimes worth that money. They do this stuff every day. I have two shops in my area that I would trust to troubleshoot, if I couldn't diagnose it myself.
There are many causes for low fuel mileage, including torque convertor clutch not locking, cat's plugging up, engine coolant temp sensor inaccurate (can make the engine run rich), a brake caliper sticking from an old hose swelling shut holding a little pressure in the caliper or corroded, sticky caliper slide pins that haven't been greased in years, knock sensors needing replacement (affects engine performance), alternator starting to fail, creating electrical noise, confusing the ECM, MAF and O2 sensors not accurate, etc
I only do this work part time, but I have seen all of these problems in the past, and they can all affect mileage.
Larry