My 06 has about 65K miles and alot of it has been pulling different types of trailers. My mileage will avergage between 9.5 to 14 loaded and about 17 to 18 empty. The D-Max seems to care more about the wind resistance than the weight. I get my worst mileage with any bumper pull trailer. My theory is that the wind comes over the cab and bed and then right into the front of the trailer. When I pull a 5th wheel the air goes over the cab and continues over the nose of the trailer. Horse trailers are the most aerodynamic and usually have a smaller frontal area. On my last trip with a 34' I averaged 13-14 mpg. I will almost always cruise at 70 because that seems to be the best trade off between speed and economy. If you want to push it above 70 it will cost you at the fuel pump.
I have noticed that the type of fuel you use and sometimes the brand makes a difference. As your truck breaks in the mileage should improve some but you won't see the numbers that a Cummins will get. (But you have to drive a Dodge for that). IMHO GM has the best cab on the road and the D-Max has to be the quietest diesel I have ever been around. Noise and comfort are extremely important when you are on the road everyday.
Actual mileage may vary depending on conditions and driving styles. All mileage listed were derived using the DIC under semi-consistant condition.