Yes, I think you should be getting better mileage. I had a 2000 F250 7.3 and I remember routinely getting right around 13mpg towing a 21ft travel trailer. This was hand calculated over many trips (no "trip computer"). Note: this was with a 2 wheel drive truck, but I wouldn't think that would make that much of a difference.
I may have missed it, but what transmission and rear end gearing does your truck have? My truck had the 3.73 rear end I think (been awhile now).
Also, has your truck had ANY "mods" to the engine? Chipped, different exhaust, different intake, etc?
Tires - I was running stock "all season" tires. If you have "off road" type tires, the tread can reduce your fuel mileage a little. Are your tires and wheels stock / original? Anything different will affect mileage. Also, are they fully inflated?
Fuel - the last time I bought diesel, it was just as the switchover to the new diesel was occurring, so I was measuring my fuel mileage using the old diesel. I don't know if that makes a difference, but usually anything government mandated makes things worse.
Driving style - Driving around town will dramatically lower your mileage, so make sure you fill up just before hitting the highway, and refill / check mileage as soon as possible at the end of the tow.