The have a strangely-worded page on the topic. I guess they're saying the Allison slips at 568 hp and some unspecified amount of torque, and the 6-speed TorqShift doesn't give up until 884 hp/1600 lb-ft (at least when managed by their tuner.)
Does their tuner not do anything for the Allison? Or did they just not include that info on the page?
I'd say you're unlikely to break either transmission towing an Airstream, unless it's an Airstream motorhome with the parking brakes on.