It seems that in the Fall (Hershey Show) and Winter (Tampa Super Show) and the spring (local shows) are when most new coaches are sold. I would expect that there are coaches traded in or for sale by private owners most months of the year. Just keep a sharp eye out for your floor plan of interest and plan on traveling to see them.
You may want to consider generating a list of what your interested in and distribute it to various dealers around the US. I'm willing to bet that most used coaches are sold before they actually hit the dealership lot. That's how we aquired ours.
Just a thought.