Selling an RV if you hold a promissory note.?

I hold a promissory note on an RV that the debtor has not paid the monthly installment for over 6 months. Can I sell the RV as I hold the title or do I have to notify them. Can they sell it if I say that's is OK to do? The RV is over 150 miles away so I cannot just get up there in a hurry. Thanks.