I like what everyone above said but the problem with their response is they missed the part that you have paid the seller.
You have a written agreement I assume, along with a copy of the cashiers check you gave up so the RV is yours subject to the lien. You are going to have to verify that he has a receipt of his lost check and mail document, try to work with him. If that fails you should go to the police, or at least suggest to him you are considering it. This could be fraud or just a bad set of events.
I would postpone the vacation until you can sleep at night. You should be able to insure it without the registration in your name, it is your RV, obviously subject to the lien.
I also think the 90 day issue is problematic.
