I own a 2013 Chevy Tahoe LT, 4x4, 48K Miles.
I just made a 2300 mile round trip while towing 2 motorcycles on an open trailer and averaged 10.5 MPG.
The total GVW of the trailer was 2500 lbs.
I have new 20" Cooper tires and all were properly inflated, fresh oil change and lube.
I would have expected at least 14 or 15 on a highway trip, even though I was towing, what I consider to be a light load for this truck.
Any idea why it would be so bad? I ran 87 octane fuel and the truck ran fine with no issues.

 

What axle ratios do you have? Makes a big difference.

 

most get 15mpg with no trailer, 2500lb is 1/2 of the tow vehicle weight... then add the poor aero of two bikes. bikes actually have a worse aero drag than a car.

 

It has the 3.42 axle ratio.