2 things I would check before going to the dealer.
1. The air cleaner. When was it last changed ? If it's been a while, I would change it out even if it looks OK. Diesel engines flow a lot more air then gasoline engines so their filters collect more junk. At least remove it and see if a mouse built a nest somewhere.
2. Is it possible you were running winter fuel ? #1 diesel will give worse mileage then straight #2. Yes, I know many places just treat regular #2 with chemicals, but some (the best ones) actually mix #1 with #2 to get winter blend.
