If the 12v battery went dead, the ECM probably lost it's memory and all the trim adjustments it stored. This means the ECM must readjust itself to optimize the fuel mileage again. This could take a few hundred miles. Check it again in a month.
PS - Wow, 5 different posts about declining MPG. Just in time for the return of summer blend fuels. The new formulations may also have a lot to do with it.