Only had one experience with an out of state purchase but trying to get a temporary registration was not an option. Went to the DMV for the temporary and they advised it would take 7 to 10 business days to process the change of ownership at which time they could provide me with a temporary registration. I was out of state that same day.
Not sure if it would apply to an RV purchase but I know some banks and credit unions will want to see the vehicle being purchased in person before granting the loan.
Might also need to consider emission requirements. Not sure what your state requires but here in CA they can make you jump through lots of hoops.