I'd back off. At this point, you are a fish to be reeled in - they've got you trying to work their price. They know you want it and will sit waiting for you to give in.
At this point, I think you have to decide:
Will you be happy if you pay what they're asking?
OR
Will you be happy if you don't get this RV?
If no to the former, then offer what you're willing to pay and be prepared to walk away.
If no to the latter, then pay what they're asking and be done with it.
If no to both - then you're going to have to be unhappy one way or the other.