That's not too bad, my trailer weighs roughly about the same as yours and with the Hemi I'm only averaging 11 mpg. So that 4.7 isn't doing to bad.
I just did a 1200 + mile trip towing about 2,600 lbs also and averaged about 13 mpg. I used a weight distributing hitch and it was like driving a freight train. No sway, no wiggles, and when you have it set up properly the rear end was the stock height with no sag, amazing. It puts the weight of the trailer tounge between the trailer and rear wheels of the truck, not on the ball hitch, great set up!
Towing a 6 x 11 x6H trailer, truck has a leer shell on it. Check to see what rear end gear ratio you have, might explain the lower mpg.