No one can tell you what rear end ratio you need on the tow vehicle, without knowing what weight you will be towing with it.
However, most newer 3500 Diesel duallies with a 3.73 rear can tow most 5th wheel trailers.
When you are looking at trailers, don't use the dry weight number to guess the weight of the trailer. Use the gross vehicle weight instead, and just assume you will load it up to that weight, because you just might.
2006 Wildcat 27RL
2006 2500HD w/Allison/Duramax
KB3TXH