You are correct in that the dual rear wheels are for greater load capacity, but you are mistaken in the ride quality comment.
The dual rear Motorhomes ride down the road very nicely, and smooth. Not to mention they tend to sway less when hit by crosswinds.
Just wanted to let it be know that WildSide is taking orders, and curently building orders for the winter rush.
If you want a rig done for next year now is the time to call.
http://www.wildsidetrailers.net