Horrible MPG need advice.
 

I have a F-250 6.4L Diesel, Super cab, short box truck with Toyo Open Country tires 275/65R20 tires inflated to 65 psi.
I drove my truck from Carlisle, PA to Frankenmuth, MI through the mountains I averaged 6.9mpg on flat terrain I averaged 8.2 mpg. I am towing a 5500# Intech trailer with a 3000 pound car. My trailer is 8.5' high.
I was reading in a car and driver magazine they tested the F250 6.4L Diesel Vs. V10, they stated that the truck pulling 8500 pounds averaged 14.1 mpg over various terrain.