Even if everything is off, there are parasitic drains such as the keep alive memory in the radio and other things like that. Also make sure if it has a heated mirror that the switch for it is turned off. You might consider a Trik L Start which feeds a small amount of current from the house batteries to the chassis battery to keep it's charge. You might also try a 12 volt ampmeter and put it around the red/positive wire on the battery and it will measure the resting draw.