Does the ignition switch have to be in the "ACC" position to unlock the steering wheel?
If so, there is your answer; with the switch in the "ACC" position, the battery will run down in just a few hours.
Many auto manufacturers, in their infinite wisdom, eliminated the "OFF, NOT LOCKED" switch position after the 2006 models. However, many later models do not have a steering lock. Does the 2008 Liberty have it? If not, there is no need to leave the key in the ignition or the switch on, and no need to disconnect the battery. I have heard that Jeep did not update the towing instructions when the lock was eliminated.