Lots of good advice in this thread, and some bad. I learned a couple things and was reminded of a couple things.
Here's my take both from school of hard knocks and learning from others.
I have not negotiated an RV from a dealer but I have negotiated a fair number of cars. You are buying a dealer and a brand. I have dealt with excellent dealerships and horrible ones. I ask around for the good ones before I start shopping. From my time on rv.net, that is even more important on RV's.
1. Salesman is not your enemy. There is a win/win deal out there. You need to find it. Goal isn't to get everything - have your nonnegotiables and places you can give in.
2. You must be willing to walk away without emotion. Not angry at the salesman. Not a fake protest, though if you are good that can work of course. You have to know there will be deal another day, maybe there maybe somewhere else.
(I make sure to buy a new to me (usually late model used) vehicle before I NEED one, ie. my current one is not dependable, so I'm not in a bind when negotiating.)