2008 F350 Ford Diesel

I am looking at buying a 2008 Ford Diesel truck and I need to know what I can expect for fuel mileage:
Highway mileage, City mileage, and towing a 31' travel trailer.
Can anyone give me information?

The company I work for just bought a F350 crew cab SRW FX4. It gets stock 18 on the HWY with my boss driven and city I'm not sure. It does get with towing. We pull a closed in cargo trailer 28' and we get 16 on the hwy. Not what else I can tell you.