SO, disconnecting the neg cable does the trick?
If not then maybe the battery is so near the end of its life that a week is all it takes for its last remaining amount of power to naturally drain out.
Batts lose power always when sitting. A god battery will not matter if it loses a little, but a near-dead one will.
If it is still draining this fast and is not an old battery then there is more than a CO monitor pulling down power. A short might do it. It could be anything, even a solar panel that lacks a proper diode.
Is there a fantastic fan that comes on when it gets hot that you are not aware of, or something like this that might be off when you are in it closing up, but then comes on when you are away?