Lost mpg, Need Help!

I have a 2001 Rodeo LS with about 45k on it. Sometime back I got a P1404 and had the EGR valve replaced and flushed at the dealer. After this I have lost about 100 miles per tank. Took it back to dealer and they say that they don't have a clue why this is happening and recommends a fuel system flush.

So, where should I start looking? What could have gone wrong?

Could be oxygen sensors (the 2 before the cat converters) They can get weak or covered with gunk and not read right but still be close enough to not set of a CEL. They can start going bad anytime around 50k miles.

I might start with disconnecting the battery for 30 minutes to allow the ecu to reset and relearn. Then I would swap out the front 2 O2 sensors (don't pay for OEM ones).

Winter gas contributing a little to the issue?

Tune up, tires, and/or lead foot?

Don't think its winter gas or things like that, 'cos I lost around 100 miles per tank!

I'll try changing the O2 sensor/resetting ecu.