Yet another person learns a bit the hard way that diesel 250/2500's are not the best for towing medium to larger 5th wheels. The 3500 model of the same truck has a 7K RGAWR and a 12,300 GVWR, and a more capable rear suspension.
OP, you can air bag it on the rear axle and pretend that it is a 3500. Enjoy!
When we bought our last 5th wheel we looked for one with a lighter dry pin weight and choose our Bighorn 3575el for its 2435 dry pin weight. Full timing with trailer at close to 16K truck at 8900 ready to tow, we were 40 lbs over the SB 11,700 pound GVWR and under on rear axle at 6700+.
The combined weight was 24,500 and with the Aisin tranny with it's lower 1st and 2nd gear ratios it worked great. Chris
Chris