My 2015 3500 just hit the 1000 mile mark this weekend, so I still don't know what to expect for mileage etc yet. The dealer gave me a full tank?? of gas, and the DEF was reading about 3/4th when I got it. Drove about 320 miles empty cargo wise, then I filled up the tank calculating gas mileage out to 16.4, (lieometer said 16.3) All driving for the next 123 miles had an 8k TT hooked up, truck computer said I was running about 13.4mpg. By this time the DEF gauge was down to half.
I swapped the TT for a fiver with about 470 miles on the truck, dumped just shy of 2.5 gallons of DEF in the truck when I got the 5er home which filled the DEF tank. I have pulled the 5er with an estimated weight around 14k (GVWR or 15.5k) for just over 460 miles. Computer estimated mileage with 5er is right around 10mpg, and the DEF gauge has gone down to 3/4 tank again. (For the weight police I'll weigh the setup camp ready in the spring as I didn't have much time to play prior to winterizing this year)
Overall with the truck the factory airbags keep things nice and level, feels very tight and secure in corners, and the truck overall has a more solid feel than the 2011 2500 Ram I traded in. The 865ft pounds of torque is also noticeable compared to last rig. It still feels underpowered when pulling the mountain passes though with the 5er back there but without going to a ridiculously large truck there isn't much I can do about that. My only real complaint is the sun roof which I settle for as I couldn't find one configured how I liked it that didn't have a sun roof installed. Being a large guy this means I have to recline the seat a little or my head hits the roof due to the added thickness to the roof liner.
For the most part, I don't use the truck for running around except when camping, hauling something like the UTV etc so other than using it every two or three weeks just to keep things from settling in to much it probably won't seem much more mileage until spring. Then, it will be time to play a great deal with both the truck, and the new 5er