There are a ton of things that can affect your fuel economy, check the basics like tire pressures, filter restrictions, brake drag, resistance in the driveline, poor engine, transmission performance, and fuel quality. Hope this helps..
could be a variety of things - have you changed your air/fuel filters lately, colder weather will decrease tire pressure and you have the winter blend of fuel as well....just a few things that may be adding up to a decent mpg loss,,sounds simple but jut thought i'd thow it out there....good luck
Winter Diesel fuel has less calories than Summer Diesel. Winter is used because it won't gell as easily in cold climates. Less calories means less power and poor fuel mileage, about 10% less mileage. This might be part of your problem.