I have a 2010 Ford F250 diesel 6.4 L, and my towing mileage is roughly 8 to 10 miles per gallon with a 30 foot flying cloud. My mileage unloaded empty on the highway going downhill with a tailwind is usually between 14 and 16. I have increased tire size by one notch and gone to a slightly more aggressive tread, which has reduced my mileage by approximately 1 mile per gallon.
The onboard computer calculates mileage usually within about 1 mile per gallon of accurate (for a full tank), however I always hand calculate my mileage and I have done so on every tank since day one. I have tried everything from a Banks performance tuner to diesel fuel additives and have seen no positive results with any of them.
This is the only downfall to this engine that I have found; it is in fact an amazing and well-built engine. The problem lies in the emissions equipment, and I agree with the statement made earlier suggesting that a DPF delete is in order. Keep in mind that this may void your warranty.