Not the Duramax, but I did switch to from a Dodge Cummins to a Ram 6.4 Gasser. Never tow heavy though I did tow the boat behind the TC a couple of times a year. Average about 20k miles/year. I meticulously track fuel usage and compare that to what I would have gotten with my diesel. Due to the diesel premium, I've actually been saving money on fuel up until this year where gas is creeping up closer to diesel. I don't really miss the power though I do miss the exhaust brake at time though not often. I'm still pulling Saluda grade at 60mph @ 3,000 rpm so plenty of power.
We are now looking to upgrade to a DRW for a heavier camper and I'll probably go with the 6.4 gas though will switch from my current 3.73 rear end to 4.10s.
Not sure what a new GM is costing, but you should be able to get a new Ram dually near bottom trim (big horn) for under $45k.