mpg on 2015 ram 2500 diesel cummins?

I have a 2014 2500 Laramie CC 4x4, stock except for a Ranch Hand bumper, tool box and headache rack that has almost 13,000 miles on it. In city traffic with too much idling it gets 15. Fast highway driving (75 to 80 or so) yields 18. I did one highway trip and left the cruise at 65 and it broke 21 for almost 200 miles. With our 10,000 pound extra tall travel trailer it will do 10 to 12 depending on headwind or not and how late we are (wife, two kids, and two dogs, so we are always late leaving). Overall (without the heavy towing) it gets a little better than my old 2005 1500 GMC Sierra CC in mpg, but cost per mile is a tiny bit more because of the price of diesel. I have been very satisfied with the mileage, especially considering the Ram with the addons weighs at least 3000 pounds more than the Sierra did.