I have 19,000 miles on mine now.My truck with full tanks weighs 10,400 pounds. It's hooked to my trailer 99% of the time. But before I started pulling with it I was getting 13.5 mpg the couple times I checked it.
I pull a 48' enclosed gooseneck trailer for my business. I transport bikes, ATV's SxS's, sleds and ??? for a Powersports auction company. My trailer weighs around 8600 pounds empty. And when it's fully loaded it can weigh anywhere from 15,000 to 18,000 pounds.
As you can imagine mileage varies. I can get as much as 10-10.5 heading east on level ground and down to 8-8.5 heading west. Most of the time I average right around 9. BTW mine has 4.30 gears as well. I don't think this is too bad considering my 1 ton gets 10-10.5 on average pulling the same trailer and loads.