Just pulled my 8K TT from home over the (small - 1000 ft) mountain range to coast. Truck is an 06 LBZ 4X4 and totally stock. Left town with a full tank of fuel and pulled back on fumes. DIC says 11.2 mpg which is usually very close to hand calculated. The range does have some decent rolling hills but I did putt around without the trailer for 40 or so miles while at my destination.
I agree with the posters that anywhere from 9-13 mpg should be expected while towing an average TT or 5th. The # within the range will depend on speed and hills. I can get 13-14 rolling down the flat interstate with no wind or a little at my back going 55. I can also get 9-10 rolling down the same interstate/same conditions at 75.