We had a 2005 Durango with hemi. It was fine towing a 26ft TT that was under 6000lbs. We later upgraded to a bit longer and heavier trailer and towing was not as good. Sorry, we never weighed the combinations.
Didn't have problem keeping the speed up in the mountains. But to do that you have to rev high in 2nd gear.
Another thing to check is that most Durango's, even with the hemi, did not come equipped to allow the max towing number. I think there was some heavy duty towing package required and when we bought ours most dealers did not have a Durango with that package.
David
2012 Fleetwood Discovery 40G
2012 Jeep Wrangler Unlimited
2019 Jeep Wrangler Unlimited