2003 Ford 6.0 Mileage Drop, help?

[Smp2010]

The other night my truck digital readout that had read 13.5mpg for the longest time went down to 11.2 and has gone up to 11.5 but not higher. I recently changed fuel filters, oil fliter, ball joints and wheel bearings(and although not that it would affect mileage poly bushed the whole truck, ugh). I thought it might have been the station i fueled up at and we had a lot of rain that week, but have since ran diesel from a station that I use a lot and nothing changed. Any ideas? Worried the PCM might be faulty. Any way I can check that without subjecting myself to a dealership visit? Any help is greatly appreciated...