A newer model 350/3500 dually crew cab with a 14000# GVWR which you are aware of.
From your earlier post
A new 2015 Dodge Ram 3500 dually diesel with Aisen auto is rated to 30k GCWR.
That truck has a GVWR of 14000# and with 4:10 gearing is capable of safely towing an 18K GVWR 5th wheel tow hauler.
Your 2006 Mega cab dually with it's 10,500# GVWR is capable of safely towing a 15K GVWR 5th wheel.
You just need to match trailer to truck.
Really want that CY4100..then get a bigger rated truck.
OR do more shopping for one that is within your current trucks ratings.
Simple math.