Totally agree with the comment above. There are just too many factors at play.
I would love to help, but it's difficult without knowing the year. Anyway, here is what I found on Hotcars.com about Ram 2500 Cummins estimated rating of 12 city/18 highway/15 combined mpg on four-wheel drive mode.
Most charts with specs will require year/make/model/trim level to get you the estimated numbers. Check this Ram chart and how many combinations there are just to look up the towing capacity