This is what I did 4 times when I bought out of state RV's.
1. Call you insurance company and get insurance and have them text or email you a temporary ins card.
2. Get an explicit 'bill of sale'. Date, time, amount of sale, description and vin of RV. AND then BOTH of you sign it. Seller and buyer.
That is all a cop needs to see if you are stopped.
They will let you go on your merry way providing you aren't an escaped convict, that your registration/drivers license are all up to date and it is not 6 months after the date of the bill of sale! :W