Many vehicles power up either the ECU (engine control module) or the BCU (body control module) or both when the ignition is in the various accessory positions to unlock the steering interlock. These computers will have a constant drain, and will eventually cause the battery to loose charge. On some vehicles it is posable, as John stated to utilize the coach system to keep the battery up. On many import vehicles attempting to charge the tow vehicle battery from the coach will cause some faults to occur in the on board diagnostic systems.
Steve & Rhonda Bennett
Dana Point, CA
92 Marathon XL40
2012 Jeep Rubicon Unlimited
Owner: California Coach Company, LLC