Are you using any of that BioDiesel added fuel? Biodiesel has much less BTU content than regular diesel fuel. When they add biodiesel to fuel to make it "green", it mostly just adds green to their bank account. You will actually burn more fuel per mile, so any "over all" pollution mitigation or "green" aspects are highly questionable.
I have a CAT turbo 300 and get around 11 MPG towing a Jeep on level ground with no wind in economy mode on the tranny. A lot can be said for large engines and fuel use. That extra power comes with the penalty of reduced fuel economy. Big engines have a lot more "internal friction" and losses than smaller engines and need more fuel just to idle.
Also watch your RPM's, diesels get the best mileage at around 1500-1600 RPM's on the highway. Hills always kill fuel mileage. I never buy fuel with added BioDiesel. Synthetic oil always helps too.
Tire pressure is one of the worst fuel mileage offenders, aside from the headache of more frequent blowouts.
If you smoke at the tailpipe, your wasting fuel. Avoid that as much as possible.