I'm assuming that you're on the 3500 Sprinter platform with the taller differential? We bought our 2013 3500 Sprinter stripped and up fitted it, and had the option at that time of going with the differential offered in the 2500, which results in noticeably better mileage due to lower rpm at the same speed, of course. Our Sprinter alone, rated for 11,500, weighs about 6900 lbs.
When we do not have the 6980 28' International behind us and there is no wind, we can get about 20-21 with the larger differential around 55-60 mph. But mostly, we figure about 19mpg for real life interstate driving with hills, passing, etc. Because of the larger differential, 2400 rpm puts us around 60 mph (if I remember correctly.) 2800-3000 around 70 mph in 5th gear. When towing, speed of the rig becomes even more critical for mpg. We consistently get 14.5 at 55, but if we want to hold 70-75, we're going to see something in the 11's. Add crosswind or headwind, and now you're low 10's.
But the rig handles great, even in 35-40 mph crosswinds (although not much fun to drive in those conditions) if we have to.
Your mileage sounds about right with your heavy Interstate.