I pull almost the same ToyHauler as you and feel there is no need for the hassel of a dual rear wheel. Keep in mind driving through the malls at christmas time, and when it comes time to replace tires (cost).
I've pulled tandum trailers a couple of times tipping the scales over 12,000 lbs and if you load the trailer smart you'll never have problems with it swaying.
Now if you were pulling 16,000 lbs or more with out a doubt I would tell you to get a dually, but 10,000 lbs is'nt that much for a diesel.
Also keep in mind that mine is a 2005 2500 dodge ram quad cab. Their is'nt much difference between the 2500 and 3500, other than the ride being stiffer on the 3500.
I have a buddy that pulls a 17,000 lbs toy hauler with a 2500 ram and has NO swaying. He is the son of a large trucking compay and has his pick of trucks and he opted for a 2500 dodge.