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I am an electrical specialist, and I will try to help you figure this out. I would check the voltage regulator first, they can be sensitive to exposure to higher voltage. You can unplug the voltage regulator, and charge the battery up, then see if the drain is still there by letting it sit like that over night. Otherwise if you have an ammeter you can see if the regulator is drawing power with everything off.
You mentioned checking the open fuses, checking the amperage across open fuses will tell you a lot more. Just set the voltmeter to amps and check that out if disconnecting the voltage regulator doesnt help.
I will be away for a few hours, I will check back to see if you have any more questions.
I would disconnect all three terminals on the regulator. If you see a power draw at a fuse when everything is off that is a good indication that there is a draw on the battery in that fused circuit. Another thing I like to do is to disconnect one battery cable, and set a voltmeter to amps. Then attach the meter to the battery, and the cable you took off. Then start disconnecting items one at a time. When you unhook the device that is drawing power, the ammeter will show that the drain has stopped.