We are towing our C Max four down. With the ignition turned to the factory required "ACC" position, no mileage accumulates. It tows like a dream, don't even know it is back there.
The one problem we incurred was that the 12V battery ran down after 4+ hours of towing. After having it checked out it was discovered the 12V connection from RV to C Max had not been connected to the battery circuit. We have a Blue Ox Patriot brake box that requires 12V for operation. I will not know whether or not the battery stays charged until our next long tow (4+ hours) in a few months.