Seems like everyone recommends a dually for stability. My rig is 14000 gross and 36' long. My srw Ram couldn't be any more stable. Passing 18 wheelers, twisty mountain roads etc. I have never wished I had bought a dually. I have enough trouble parking a srw. Why would you want to buy six tires when they need replacing.
The exception to this would be if your rig is so heavy that you need the weight carrying ability of four rear tires.