12.1v is the voltage when the battery is 50% charged. As long as the current drain is small the voltage will not be drawn down much. I never disconnect my battery to determine the exact 50% point, although I could just use the battery disconnect switch. The exact 50% voltage is temperature dependent anyway and that is a variable you can't control.
If you really want to know what the residual draw is, get a clamp-on ammeter with a low range setting. Of course you can do it as you describe, but is it really worth the effort?
