I've been towing with Cummins 5.9L & 6.7L diesels since 2001. I've never spent a cent for anything other than routine maintenance, nor have I ever been stuck on the side of the road.
Why did I go from gassers to diesels? Our 1996 Ram 3500 dually (8.0L V-10/47RE 4 speed automatic) got 6.5 MPG pulling our 2000 Jayco Designer XL 3610RLTS on a given route through the Texas Hill Country. Pulling the same trailer over the same route, our 2002 Ram 3500 dually (5.9L Cummins HO/6 speed manual) got 11.5 MPG. On the 6% grade on I-10 into the east side of Kerrville, the 1996 was in 2nd gear at 3500 RPM to hold 55 MPH when towing. The 2002 would pull the same grade in 6th gear at 65 to 70 MPH towing the same trailer at 2200/2350 RPM (4.10 axle).
Here in Texas, you can hardly give a gas engine dually away at trade-in time, whereas the diesels will bring a premium.
To each his/her own, but a diesel (especially pulling the 5th wheel we're towing now) makes perfect sense for us.
Rusty