I have had 1997, early 2004, and now 2013. All of them got/get 17-19 highway solo with some mountains, 75 mph. All got/get 14-15 city, 11-12 towing a big frontal area trailer that weighs 4000-8000 lb, about 13 towing an open trailer with a car on it, about 15 highway with the open trailer empty. With more timing, my 215 hp version of the 1997 could get around 20 at 65-70 mph. I haven't tried that speed with the 2013 but it got around 19 at 75 on I-40 which has some elevation (up to 7000 ft).
Some will claim better, but often when asked for details, they are at or near sea level, on the flats, and driving gently at 60-65 mph. Out west, the destination is too far away for that, etc. I feel that my 2013 while heavier than the other years I had by 500-1400 lb, will do as well in the real world as them. The torque curve of the new 6.7L and the 3.42 gears work well together.
The TDR has said for about 20 years that you will lose about 1 mpg for dually, or for 4 wheel drive. You will also lose mpg for a lift kit with big tires, high speeds, and sometimes for a huge aftermarket front bumper.