I had a 2013 with the 6.0 and 4.10 rear end. I towed a 28ft fifth wheel and was satisfied with the results. It was necessary to wind it up when going up a steep hill, but you just need to accept the fact that the motor will run at a high rpm all day long.
I just got a 2019 3500 with the 6.0 and 4.10 gears. I now haul a Northstar pop up and tow a utility trailer. I love the motor. It just goes!
The 6.0 does NOT have cylinder deactivation
Good luck with your purchase.
Bill