A 2012 Ram 2500 diesel is still a baby, hardly broke in yet. If it was a 20 year old truck, there might be some advantage. With a diesel, a 20 year old truck has finally "arrived" (engine wise), but the body may be rusting out (which happened to us). But your truck is too new (in my opinion) to trade it, especially for a gasser. I think if you got the comparable truck as a gasser, even a 2015 model, you'll like the 2012 diesel better and be kicking yourself.
Now, if you are experiencing problems with your diesel, looking a gigantic repair bills or something like that, or if the body is failing .... well ... then trading might be a good thing. But to trade for trade-sake only, or fuel economy.... nah! The depreciation hit will kill you, more than the price of diesel vs gas! Keep the diesel for another 10 years. You've got a good truck right now!