You do NOT have to get a in-transit tag. Those are given out by dealerships and DMV as a state 'temporary' tags.
You 'register' your vehicle for an in-transit tag thereby you pay the taxes. You do NOT live here so why would you register it here and then have to change it again in you home state.
Private party, no taxes paid until you get plates for state you live in. And depending on the state if it is used the tax% is different that the taxes on a new vehicle.
Buy it, get a signed by both parties receipt and go home and register it!!! :B
BTW, congrats on your new RV! :C