Hi Dubldeck,
Welcome to the Heartland Owners Forum.
Model years generally change in the spring - but varies by model. By the fall, when most people view camping season as ending, dealers will want to clear out old inventory. So you may find dealers offering special pricing and being more willing to deal. But of course the availability of a particular model at a particular dealer is problematic.
RV Shows are another possibility. The manufacturers often provide incentives to the dealers that can result in very attractive show prices.
When looking over a particular trailer, look on the frame, toward the front, for a manufacturing label. That'll show you the date of manufacture, which will tell you how long the dealer's had that unit on their lot. That may give you some insight into their willingness to deal.