I expected to MPG's to go down, but....!

[JYaekel]

I am new to the community, as well as new to my 98.5 4x4 auto diesel. I recently hooked up my utility trailer with pressure washing equipment on it and took it for about 150 miles to a job.
Well, I was rather shocked at the drop in the mpg's!!!
First off, my trailer with equipment only weighs about 2500 lbs (and I think I am being very generous with the weight). It does have a 4' Ramp gate on the back for loading....
With no load on the truck, I am getting between 18 and 19 mpg doing between 65 and 70 mph. I figured for a 4x4 auto this was about right..... After towing with the trailer, however, I am getting about 10 - 11 mpg tops!!!. Being as this is a light trailer, I did not expect the truck to even notice the load much less show that much of a reduction in effieciency. Driving habits were kept the same between no load and loaded mpg test.
I am wondering if the tail gate (though it is of metal mesh) could be adding to the problem???

Any thoughts and advice from you pros would be greatly appreciated.