I tow a Ford Flex. When I hook up I just put the transmission in neutral and turn the key as far to the left as it will go. On my vehicle, that turns off all accessories (including the radio and A/C), but leaves the 12v plug active. The steering is still unlocked and I have never had a problem with the battery, even towing as long as eight hours. My son in law has a Ford Escape and follows the same procedure and has never had a problem either.
You might give that a try before adding a charge line.
Please let me know if it works for you