Yes, the RV can be taken across the border into the US, as long as the owner is aware and OK with it.
The renter should print the booking voucher, the roadside voucher (if selected), a copy of the RV registration, as well as the insurance certificate that the owner will provide them.

Our insurance and roadside assistance products are valid in the US except for some prohibited areas.
For details regarding the prohibited travel areas, please refer to the bottom section of the Terms of Service.