Consider this. You buy the RV, insure it, and on the drive home it’s totaled. The insurance co. is only going to give you Blue Book regardless of how good a condition it is in. That may be a small risk but are you willing to accept that risk? Secondly, it’s 20 years old - that’s reality, things happen simply due to age; that’s part of a Blue Book value. What is it really worth to you?
Personally, I’d offer Blue Book, if they decline, I’d give him my telephone number and tell him the offer will stand if he changes his mind.
My personal policy is to never knowingly over pay for anything. I work to hard for my money to do that.