With a fifth wheel @11,000 you will be putting at least 2,000lb on the rear axle, what is the rear axle unloaded? Air bags will not take away that mooshy feel and could possible make the truck bounce more, the sweet spot for air bags is to air them up to the point that your overloads are just engauged. This should put you pretty close to level, but if your springs are just too overloade you would be better off upgrading your springs. I tow a gooseneck with a gross of 13,000 with a 2500hd and I was over on tires and the rear was mooshy just like you describe, I did bags and it did not solve the issue.
