I've personally towed an 18K lb trailer with my 01 (3500 dually)... I felt the trailer didn't have good enough brakes to make the tow a great tow... and the truck was great... a little low on power... . never overheated on us...
I currently tow a 22K lb trailer with my 04... the trailer has power disc brakes and the extra power of the 04 make the job an easy one... . never felt I was not in control... . more on this later as we go through the first winter..... I now have an 05 on order and it was built yesterday if they kept their schedule... . so it should be on the rail car later this week.....