Do I need extra insurance when I borrow friends RV?

If you are borrowing a vehicle and the owner can add you to their insurance policy, then that is all that needs to be done. Insurance follows the vehicle, not the driver, so the owner of the vehicle is responsible for any accident that may occur during the use of the vehicle by another driver. If such an accident occurs, the owner of the vehicle will not have their rates affected. The driver who is at fault will have the accident on their record and the rates for their insurance could go up.You cannot insure the vehicle as you have no ownership of the vehicle, so there is no insurable interest in it. Your agent should be able to confirm this for you. However, if there is a serious accident and the insurance on the vehicle is not sufficient, your insurance could come into play for the settlement, presuming the accident is your fault.