1. Decide that the unit is worth driving to see (pictures, conversations with seller, etc).
2. Decide that you want to buy it (check it over and test drive and likely have it inspected by someone familiar with your choice).
3. Agree on price, closing date and get a copy of the title showing clear title in their name. If lien, you will be dealing with their banker-- not a problem, but changes how things will work.
4. Safest way to protect both buyer and seller is to wire transfer money to the seller. My suggestion is on closing date to physically go to where the unit is (make sure all is OK). Then, contact your banker (I always pre-arrange with my banker so they expect my call) to wire transfer money to seller (your banker to his banker if you are more comfortable that way). You take possession and title once seller confirms $$ in his account (usually less than one hour).
5. Paperwork needed depends on where it is registered and where you will register it.
Have done exactly this process both buying and selling expensive motorhomes and boats.