New from a dealer you can expect 30-35% off MSRP. Once you find a unit you like, shop around - like all over the country (online of course) to see what the best deals are. RVDIRECT.com - if they have your unit, usually has prices about as close to the best deal you can get. Even if you don't buy from them it's a good benchmark to leverage against your local dealer. I always told my dealer that I would never have an issue driving a few hundred miles to save a few thousand. They usually come close.
Now on used, you just need to shop on line to see what other owners are selling the same unit for. A lot of folks are emotionally attached to their unit and have unrealistic expectations. But there is no hard and fast rule for discounts on used RV's. It becomes a dance of finding similar units in similar shape with similar miles as a guide. NADA can be helpful.