I say anytime you think you are getting a deal. Many times after RV shows dealers will have a number of used. I bought used through Craigslist. Unless cash is not an issues, I have a really hard time to buy new big ticket items, Houses, Cars, RVs. The later are deprecating assets, why not let someone else take the major financial bath the first couple if years.
Others can chime in on this theory, but I think the best way to know if you are getting a deal is to research craigslist, ebay and NADA/blue book, and RV dealers. You should be able to get a rough idea, maybe even find a few no haggle dealers with rock bottom pricing.