First, you need an address that is recognized by SD as a legal address. SD has several mail forwarding services, most (if not all) of which will work. We use DakotaPost
in Sioux Falls. They have someone on staff who will actually go down to the county office and process the paperwork for you for a fee (somewhere around $30 if I remember correctly). Their website has more info on the requirements for vehicle registration as well as driver's licenses and voter registration.
You will also need vehicle insurance that is valid for SD. We use Miller Agency
. They are very good with full-timers and can provide quotes from several companies.