A lot of good replies and information provided. Glad to see you are knowledgable on weight ratings. I also think a dually will drive sooooo much better with a load, especially in wind or mountains.
My truck; 2011 Dodge 3500 dually quad cab 4X4 diesel (6.7), automatic. Mileage 10-12mpg. Now those numbers are pulling a 14.5k fiver, I get around 13mpg with the TC (2500 gross). I tow at about 60 to 65 max mph (again, read the tire side wall on trailer tires). Camper, usually drive the speed limit unless it is insanely high like out west or Texas.
For me, the hardest choice about the truck was what gearing for the rear end. Dodge offered a 3.50, 3.73 and a 4.10 gear ratios if I recall. I finally decided on the 3.73 and it has worked very well with the automatic transmission gearing.
BTW, make sure the truck has an exhaust brake. It is unbelievable, you won't believe you towed without one!
Cheers