Are you testing the voltage from the negative posts to the frame with the negative cables connected? If the cables are connected and stuff is turned on then the .9vdc you're reading indicates you have high resistance in the negative cable or a bad/loose connection at the post or frame.
In any event this is not related to the battery draw.
If your meter has a 10amp position I would connect the meter in series with one of the positive posts and check for draw by turning everything off. Check that reading and start pulling fuse to see if the the amp reading drops.
If everything is off or disconnected then I would suspect there is a wire rubbed through and has a high resistance short in the body. Check for warm wires at the fuse block. A dead short would show by a burnt wire.
Hope that helps to get you started finding the problem.
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