Is my mileage unusual?

NO, 13-14 is NOT a pipe dream as we have been towing our Carriage CarriLite that scale weighs 12,800 lbs when wet and loaded to travel and usually have our 106 gallon aux diesel tank in the bed full. Have towed it for many years with our 2004.5 GM 6.6 D/A CC LB 4X4 truck and 13-14 hand calculated is what we've average constantly on over 60,000 miles so far with the rig combo. Always use the cruise control and drive at 58-62 on Interstates and 55 on normal highways. Speed is the biggest killer of MPG's especially when towing heavy. Most people we know that have the Ford 6.0 PSD get roughly 3/4- 1 mpg less towing their 5'ers than we do when we go places together. "How many gallons did you take" is always a topic after refueling.

9.8 MPG's best is very low and I'm sure the OP can figure out why he gets that mileage assuming his truck/engine is running properly.