I have a 2013 Durango Crew as well with a 3.6L. From what I've found, towing anything more than 1000-2000 pounds, any little up hill run will cause the tranny to down shift. I found it hard (in the sense of the constant downshifting) to stay above 60mph.
I currently tow 5000# and I manually do the shifting and at cruise I stay in 4th. I find the manual shifting stops the computer from chasing gears. When I hit any little hill, I will have to downshift to 3rd which kicks rpms up to 3-3.5k to maintain speed.
For me, at 65mph, my engine runs just under 2500 rpms which is also the rpms at 75-80 in overdrive.