It's very easy to "haggle" on a new unit... not so much on a used unit.
When we bought new, we just got a quote from RV Direct, added $ 2,500.00 to that price, and took it to the local dealer. After some back and forth, and the dealer throwing in some options, we agreed on a price, and that was that.
There is an old saying that the value of a used vehicle is whatever someone is willing to pay for it. About all you can do to prepare is get prices on units of the same make, model, year and mileage, and come up with an average price.
Then, decide what the most you are willing to pay, make the offer, and stick with it. The dealer may throw in some accessories to sweeten the deal, which is fine if you're still OK with the final price.
Just be prepared to walk if you don't like the price, or if something just doesn't "feel right". Remember, that may be the start of a long term relationship with that dealer, and you need to be comfortable with it.
Good luck, and happy camping! :)