I live out west and towed a 5,000lb loaded TT for 10 years with a 2004 5.7L Durango. Traveled numerous mountain grades in AZ and CA. On the steepest grades I was down to 35mph. Not because of lack of power but because of keeping RPM's reasonable and not pushing it too hard.
The biggest issue is towing these grades in the summer when its hot out. The Durango came with a factory auxiliary radiator with fan that would turn on when engine got a bit hot. It quickly cooled temps back to normal. So depends on when and where you'll be towing. A good radiator and possibly transmission cooler is recommended.