I have to say it depends (what a cop out, eh?). If it's the end of season, the trailer has been on the lot for a long time or it has received some sort of cosmetic damage (eg I saw an ad one time selling hail damaged trailers at a very good price - they were perfectly safe but had little dents all over the place), the dealer is likely in a good frame of mind to consider offers. On the other hand, if the trailer has just arrived or that particular make of trailer sells fast, then the dealer won't likely be so willing to accept below their price. Also, if you are paying cash that may help get you a reduced price. For some dealers, the offer of cash on the spot is incentive to lower the price as they won't have the extra work of setting up financing.
When you go to look at your potential trailer, watch for blemishes, deficiencies and what not - point those out to the dealer and get those fixed as part of the purchase price or request a discount on the purchase price if they can't/won't fix them. Remember to see how this impacts the trailer's warranty as well.
Must say I've never heard of factory rebates on trailers but I haven't looked into that closely neither.