Phil, My wife and I just bought a 2015 Forester also. We have the Blue Ox system but the issues are the same. It tows excellent, yes you have to leave a key in the ignition with it turned to ACC, 1st click. Make sure lights and radio are off. Turn off the dash display.
With the valet key in we figured out how to lock the doors and we have no battery problems. But remember only in the ACC position and not the ignition position or you will draw the battery down.
Safe travels.