As for the lower MPG in winter in Pennsylvania - could be several issues here, only indirectly related to temperature:
1) Winter diesel formulas, which, everyone seems to be certain of, are less energy dense than summer formulas.
2) I've been to Pennsylvania during many winters: lots of snow. Snow on the roads results in more sporatic driving - more stop & starting, driving in first & second gear, etc - which may lead to lower MPG.
In response to another post: AC uses a hoopload of power. I've done VCDS logs to determine fuel consumption of AC at various speeds - .6 liters/hour, at whatever speed. I also ran the numbers as MPG at various speeds - with/without AC - my logs showed a much bigger hit than 3 or 4 mpg - if I remember correctly, more like 6 mpg - but that varies. Consumption per hour doesn't. Of course, I'm in Arizona - so, when the AC is running, its running full-on.