Well I know they upgraded the axles to the AAM models, my 03 3500 4x4 the front outer U joints are larger than the rear driveline ones on my 1976 Jeep honcho... The new 2013 frame is made of 50,000lbs. tensile strength steel , they strengthened the transmission(s). The gooseneck 5th wheel mounting setup is integrated at the factory, I do believe that when the 2003 3500s came out they did a new duty cycle test of loading the trucks to 95% of their rated capacity and then running the trucks to the equivalent of 150,000 miles in extreme temperatures... hoping that type of testing has been done every year since.... I'm sure Dodge has done some other upgrades to the suspension and brakes ,engine cooling...
Still don't know about descending a large inclined hill for miles with a 30k load on any one ton...I am with you about not being comfortable pulling and stopping a trailer that out weighs the truck by over 3 to 1... I do believe in long term durability testing and under rating to increase the safety factor. and it is very possible (likely) Dodge has done just that, would just like to see it in print...