You've started right by doing some of the negotiating vie email.
When doing that in person, emotions can get high, salespeople get pushy, and I sure get annoyed at the back and forth to the sales managers office, with them bringing out numbers scrawled on paper with a black magic marker.
Go to the Hershey RV show. Let the local dealer you've worked with cool off, like others have said.
All the dealers pay the same price for the unit. The only variable is transportation costs - but with the dealers on the east coast being so far away from the factory, the transport cost differences among the dealers in your regional area will be minimally different.
Another idea is play several dealers against each other. If your preferred dealer still won't budge, tell them DEALER X has agreed on this price.