Paying dealer prep on a used motorhome?

Their profit is already built in. An additional half grand is not something I would pay. That was tried on me. We negotiated the price and when I went in to pick it up, they added a "dealer prep fee". I told them no, walked out and went home. They were on the phone to me within a couple of hours wanting me back, without the fee. I nearly walked out again when they wanted to charge me for a second set of keys. Again I told them no and started to leave. Lastly, it was an extended warranty. I told that guy that he was betting it would not break down during the warranty period and I never bet on another man's game. They finally gave up on anything other than the agreed upon price. They will try anything for a buck. This was not on my most recent m/h purchase.