||I am towing a 4,700 lb (when loaded) 24-foot Fleetwood Wilderness travel trailer. I am using a Reese weight distributing hitch and Reese dual cam sway control system. It tows well and Hp/torque is certainly not a limiting factor. I can pull a 6% grade at 65 MPH under 4,000 rpm - not that you would want to do this routinely, but I wanted to test the envelope.
The suspension and payload capacity was my limitation however. The payload limitation on my 2WD SR5 is about 1400 lb. I assumed a tongue weight of 12% of the trailer weight = 560 lbs. Plus 500 lbs of passengers; 100 lbs cargo; 120 lbs hitch system - totaling 1,300 lbs. I then considered myself to have a 7% safety factor - something I am comfortable with given the expected performance of Toyota products. I am however, considering a suspension stiffener such as air bags just peace of mind.
Hope this helps