Sounds normal but is it really a problem? What percentage of your miles are towing vs your other use of the vehicle? If you did a 1000 mile trip at 10 mpg vs 13 mpg the difference would be about $47.00 Not enough to worry about if the vehicle does everything else well.
As donno said, your axle may be a big part of it. What is the mileage when not towing?
I had a 1999 Silverado plain Jane truck with a 4.3 and ran the snot out of that thing for 225,000 miles. I did replace the intake gasket and water pump at 160,000 but it was a fantastic engine. It still ran excellent when Obie gave me $3,500 on the cash for clunkers deal.
I also had a 2013 Silverado 5.3 with a 5,000 Jayco TT that got 10 or 9 at times.
I now have a 2014 CTD that's doing 22,000 gcw and it gets 10 to 11 but empty at 60 it gets 25