Thats a hard one, because there is very likely nothing wrong with the truck.
Here's pickuptrucks.com towing mileage on the ecoboost. http://blogs.cars.com/files/mileage-chart.pdf
I know they are running about twice the weight, but what you are pulling probably has more wind resistance.

This the the problem with turbo engines, it can get better fuel economy empty, but when you really get into the turbo, it will use the gas.