The neighbor in the storage facility that I use has that same truck....Freightliner with the sport chassis (custom bed with side storage etc.) Talk about a nice hauler..air seats, all leather and just about any option you can think of installed. I had a few chances to "tour" the unit and talk to him about it. His has the 400 hp cat @ 1050 ft lb torque and the Allison six speed. He tows a Teton Reliance 5er which weighs around 24K lbs. He says it is a joy to tow and has absolutely no problem getting over the mountains. He is giving his wife driving lessons so she can take over some of the driving chores (freeway driving only according to her). IF you are going to pull something as large and heavy as the Teton you might want to put this rig at the top of your list. He has 38K miles towing without problems and the unit is just now getting broken in.
He has to store both units in the storage lot (covered)..I'm just glad I don't have his storage fees. ;D