Very happy with my 2008 towing a 18' tongue to tail #4500 dry trailer. We have had the trailer up to #6000 for cross country trips, but usually around #5500 here in Colorado for weekend trips. We always travel with a full water tank and dump at home after the trip. We have been through the Cascades of Washington, the Rockies in Colorado, the plains of Kansas and the Appalachians out to DC. Even driving into 50mph headwinds in Wyoming, the truck never had an issue, other than mpg dropping under 6. We have a Hensley arrow and have never had any tail wagging the dog issues despite some panic stops and many miles of curvy mountain roads. I wouldn't want to pull much more trailer than I do now, however, because I like the fact that it doesn't appear to be too taxed by my set-up. We are under all ratings at the scales.
Currently have 60K on the engine. Blackstone tests come back below universal averages every time for wear indicators and 40K of that is towing miles.
I would definitely stay away from the 4.6 ( if they still offer it ) and get the towing package.