If they have clear title to the RV and it shows the same address as where you are meeting them there is not going to be a problem. The risk is more on the side of the seller in terms of a buyer bringing a cashier's check and the seller not knowing if it is legit.
There are also escrow services available that are not terribly expensive and a good way to protect both the buyer and the seller, especially if they are not in the same state.
I would worry most about an undisclosed defect with a used RV and some states have great laws to protect the buyer and others, like New Jersey provide no protection at all. So it would pay to research the consumer protection under the law in your state and to have an independent inspector go over the RV.
I would trust my gut (and experience) in evaluating a seller or a buyer as to their trustworthiness and if I had any doubts at all I would walk away in a heartbeat.