Just returned from a trip to Yellowstone from KCMO. Roughly 2300 miles round trip. To be honest, trailer weight has nothing to do with MPG. It's all about wind resistance. Two years ago I towed a little 5,000 lbs TT on the exact same trip and got the exact same mileage compared to the 5ver I now have. (Check my setup in the signature) Both of them stick up above the truck cab.
So here are my numbers:
City: 13-15. Depending on the number of lights I have to go through.
Highway: 18. That's open interstate at 70-75 mph.
Towing: 9-10 against the wind, 11-12 with the wind.
On a different note, towing through the mountains around Yellowstone, this truck is a monster. It eats steep inclines for breakfast and asks for more. I couldn't have been happier with the performance.
Hope this answers your questions.