My 2012 VW Touareg 3.0 TDI has very similar power ratings to the new Ram 1500 diesel, and also has an 8-speed transmission. The Touareg has a 7,700 lb tow rating. I'm towing the trailer in my sig and averaging 14.5 MPG at 60 MPH. Unloaded I'm getting 29MPG at 78 MPH. I don't think the Ram 1500 will be able to match the unloaded numbers due to additional wind resistance and weight. Hopefully it will exceed the loaded number.
I'm not sure why the loaded MPG's on my Touareg are only 50% of the unloaded, but it seems like that could be improved somehow. My experience with turbocharged engines has been that they are great on gas mileage when you stay off the turbos, but towing on the turbo 100% of the time kills it quickly.
I'm glad Ram is doing this, but I don't think it will make much sense as a dedicated tow vehicle. It will probably appeal to the people that use them as daily drivers unloaded. Our Touareg works very well for us because we put about 27,000 miles a year on it but only 4,000 towing.