Obvious plus to used is saving money. Downside is lack of any warranty, even the aftermarket ones usually have lot of clauses so be aware. A good inspection, not by the dealer, can find some obvious flaws as long as you're not blinded by having that new toy sitting in your garage. There are some Pre-Deivery Inspection forms out there that can help, or you can hire an outside person. Once you shop around you'll find the average price for the one you are looking for and that helps spot a good deal or somebody that is trying to off load one for other reasons.
Many, dare I say most, owners are upside down in their RV loan so many are not looking to deal.