There is nothing to worry about in regards to the IRS, they don't care what you do with your money. It's the Department of Treasury that tracks those transactions looking for money laundering and/or terrorist activities. Most people misunderstand how this works. Paperwork needs to be filed when you deposit or withdraw ~10k in a day, doesn't matter if it's lump sum or in multiple transactions. But that's not where it's limited to. Different types of transaction have different type of reporting. A transaction as little as $2000 could trigger reporting, or if a teller/bank feels suspicious.
Bottom line is, if you're not doing anything wrong, you don't have anything to worry about. I used to regularly make ~$10k transactions multiple types a month out of my personal accounts for business and personal(buying cars). Only issue I've ever had over the years is a bank wire denial and that was because Chase thought I was being scammed for $9k. I went in to the branch and explained it was to my business partner and never had a problem again or heard anything from anybody.
ETA: just take cash. Who's going to know you have that much money in your car besides the two of you? When buying a car, I would withdraw what my max would be. Then I would separate the money before arrival for negotiating. First group would be my low offer so they didn't see I had more. If they wanted more, I had smaller groupings I could take out to make it look like I was tapped with that next offer. Or I could act like I need to go to an ATM even though I had the money.
I don't understand why you're not just writing a check. Paying with cash is dangerous for a number of reasons. If the RV seller is wary of accepting a check from a stranger, that's understandable, but you can write in your sale agreement that he'll send the pink slip upon successful cashing of your check. I mean passing whatever his bank's time-frame is for making sure a check is good. That way you both have something on paper documenting your agreement, you take the RV, he cashes your check, he sends you the pink slip, done. Plus that way you have your canceled check as proof of payment, which can come in handy later.
Cash? I would never even consider it. Don't forget that there are all kinds of scumbags out there, including people pretending to be selling vehicles--and requiring payment in cash. The 'buyer' shows up innocently expecting to look at a car, and goes home empty-handed after the 'seller' (and possibly a few of his friends) rob him. I'd avoid cash at all costs.
