I wouldn't say I'm the best negotiator, but I've only failed to buy a vehicle at a price I was willing to pay once and it was mainly because it was a limited availability vehicle. They weren't willing to negotiate and I wasn't willing to pay their price. We shook hands and parted ways.
I"m always honest with the salesperson. If they ask for information I don't want to give them or don't think they need at that point, I will tell them that, note make up something or excuses. If I'm only looking with no intention of buying immediately, I'll tell them that right up front. If I'm going to purchase immediately or very soon, I will tell them that as well. I'm never rude (even when lied to) because you lose credibility in my opinion. I will and have called salespersons when they were "mistaken", but still been respectful about it.
I'm a bottom line negotiator. You put whatever numbers you want on the contract, I'm only going to be concerned with what I've got to fork over. I research and estimate as best as possible what the dealer has in a vehicle and then decide what I'm willing to pay, out the door, for the vehicle including taxes and govt mandated fees. I always have the salesperson give me their price first and go from there. We've literally bought cars in a matter of minutes (not including all of the paperwork) because I was happy with the price early on. I know people that are proud of the fact they spent hours and hours or days and days to get a few hundred dollars off of a vehicle purchase. My time is worth more to me than the few hundred dollars.
I think the biggest problem most people have when buying something like a vehicle or an RV is letting the purchase become personal. It is a business transaction and should be handled that way (thus the not being rude rule). When you let it get personal, you will almost always be on the defensive in the negotiation.