Toyota Repair: gas mileage, rich fuel mixture, gas mileage

Question
I have a 2006 Camry LE, 5 speed automatic, V4, with full factory warranty (7 year/100,000 miles) and it's also certified pre-owned.  I bought it at the beginning of March with roughly 43,000k miles.  It now has 53,785  I recently experienced a dramatic drop in gas mileage and I'm puzzled.  I am careful to complete all factory major/minor premptive maintance at the right time.  Previously I was getting 27mpg with a mix of in town/highway driving, but in the last 3 fill ups that's dropped all the way down to around 21mpg.  The only recent changes I've made were to inflate my tire pressure to the correct level and buy a brand new battery (one I had inexplicably died).  What could be causing this?  I calculate my mpg at every refuel and this drop was sudden, not gradual.  Thanks, E

Answer
The best way to go about this is to have the toyota dealer connect the techstream scantool to get some data from the computer, I doubt that anything will be found because a overly rich fuel mixture would turn the check engine light on. If they do find something it's probably still under the warranty.