You do not check for battery draw (current) by measuring the battery voltage. You need to use a multimeter to measure amps. If you disconnect the battery ground cable and attach one lead of the meter to the ground terminal of the battery and the other to the ground cable you can verify how many milliamps are being drawn when the switch is off. Make sure you have the meter set to read current and not voltage or resistance. You can then pull your DC fuses one at a time and check for any variance to identify what is causing the current draw. If none of the fuses make a difference then you probably have something hard wired to the system like a propane detector or radio that is using current to maintain memory. The radio may have its own inline fuse which is not located in the fuse panel. All reading 12.33 volts tells you is that you have a patially dischargred battery which can be caused by current draw or having never been fully charged.