sounds like you should figure out what codes 25 and 26 are....and start there. Also, if the mechanic was right about the Knock sensor, you may want to look at that a little closer....like check for a frayed or broken wire very near the sensor....A knock sensor that is preventing the timing from fully advancing could cause poor power and bad mileage.....yet it will idle just fine and will rev fairly normal under no load.

edit------I could see codes 25 and 26 being thrown for a condition where an engine under load with retarded timing is wasting gas....incomplete burn....causing the oxy sensor to send a LEAN request to the ecu....

I bet if you look at the plugs they are black and sooty.
You forgot to tell us about all the backfiring it has been doing.