I have towed hybrids for over 5 years now (4 years with a Mercury Mariner and 1 year with a Ford Fusion). I have always turned the key all the way back towards the OFF position, not ACC. Have never had a problem with battery drain - - or any other kind of problem either. Why would u need to put the ignition in ACC position?? I have a Brake Buddy and plug it into the 12V outlet which is always "live" even when the key in not in ACC position.
On long straight trips from point A to B I have left the toad set up that way for as long as 3 days while using pull-thru sites in campgrounds (or overnighting at Wal-Mart) and never had a problem with the battery running down.