When I was in the car business and selling to a customer out of state,,I would call the tax office in the county in which the vehicle would ultimately reside. They would give me the tax rates and any other info required to legally register a vehicle.
We would do,the loan with the tax/reg info and costs included in the loan and just send it to their county/state of residence. I can't imagine it would be any different with the sale of a RV.
Call your local tax office, let them know what you want to do and they can give you all the info you need.