Holding loan for buyer

Bad Idea, 1st time there is a problem with the RV , they will want to use monthly payment to make repair or just stop paying, then you must hire a lawyer to sue them and you may or may not get your money. But you will have to pay the lawyer.

Bad idea. Cash talks.
Loans are bad enough with friends and family.
Years ago my wife's good friend wanted to borrow $5,000.
I said "Okay give me the title to your car."
She paid us back on time.

You'd be surprised at the number of boilerplate legal agreement forms you can find at Office Depot or Staples. That's where I'd start.Then of course, there's lots of legal form companies on the internet: Legal Forms

I would highly recommend against doing it. You may never see your money or motorhome again. Good luck!