I just purchased a pre owned RV in Oregon. I live in California. I just sent all the paperwork into Clay County South Dakota. I am paying them the sales tax of 4% plus 10 dollar title fee and 220 registration. Clay County Treasurer has a web site. Being from Ca. I almost went into shock when they answered the phone and are very friendly and will answer all your questions. I have called a couple times always a pleasurable experience. The best part is they will mail your plates to anywhere you don't need an address there.
Now I just have to stay one step in front of the revenue collectors (CHP) till I retire in December and head south.