My 2014 Chevy Silverado 3500 Duramax long bed, crew cab, dually diesel gets around 13-14 under normal average driving conditions. On interstate long distance, it will jump up to 15 on flat ground.
Towing my 10,000 pound travel trailer for 4 years, I consistently got 10 mpg. (occasionally got 11, but 10 was the average)
Towing my now 12,000 pound, loaded 13,500 pound fifth wheel, I'm getting 10. In hill country it will drop to 9-9.5. In the mountains, it dropped to 8.5.
It's not a 3/4 ton, but maybe it's something you can compare with.
Another thing I noticed, especially towing my new 5er is speed. Using the DIC (Digital Information Center) on the console, when driving 55 mph I'm getting 12 mph. When I pick that up to 60 mph, I'm getting 11. When I kick that up to 70, it drops to 9.5 ... consistently.
The sweet spot is 60 mph at about 10-11 mpg. Any faster, and the fuel consumption really begins to jump. And remember... this is a 3500 dually towing a 13 foot, 4 inch high fifth wheel.