I also worry about the DEF system screwing up. This year with near $6 diesel, I haven't driven the 3500 much, just a few times up to get loads of stone for projects. My 2015 RAM 3500 Cummins runs out of Maxcare coverage next April. I initially planned to replace it with a 2023 but to get what I want in trims, I'm looking at 90K in 2022 dollars.
I have never had issues with the powertrain on the 3500 and don't expect any issues even with the DEF system. From what I understand from reading RAM forums, it is easy to pull the DEF injector and clean it if it does crystalize.
Hay Dude, The ECO is a fantastic daily driver and it does get average miles per gallon in the low 30s to upper 20s. But the Cummins, in the same driving style and area I drive in is getting 24 MPG average not pulling. I was thinking, that the Cummins is better than twice the displacement of the ECO, and my 3500 weighing in at 2,800 more pounds than the ECO, the ECO should be doing better with the MPG.