A few simple rules I follow myself in MOST situations when buying any Vehicle.
YOU must be in control of the process.
I make it clear with my body language, my demeanor and my language that I am in control of this transaction NOT the salesman. I am always polite, but firm, direct and make it clear I don't suffer fools gladly.
This is in direct contradiction to what most vehicle salesmen are taught. This is why they try so hard to get you inside the building and sat down in a cubicle or office.
Know what you will pay before you go.
Make them an UP or DOWN offer. (an offer they must accept or reject. No further negotiation) And make it clear to them that if they try to negotiate it you walk. Nothing will focus the Sales Managers attention or get him in your presence faster than this. And then you follow through.
If more people dealt with dealers this way a lot of the bad behaviour we all see from salesmen and their managers would disappear. And to be fair some Dealers, Car as well as RV have changed their tactics to a much more customer friendly, easy approach.