[Barnsnake]

Be sure to use a device known as a battery isolator. This will allow the "towed" battery (Cobra, Boat, or RV) to be charged by the tow vehicle, but will not allow the tow vehicle to "use" that battery for its own needs. They allow the use of lighter gauge wire since there is no danger of the tow vehicle trying to use the "towed" battery for cranking power. They are available from marine or RV dealers.