Marshall,
Using a VOLT meter in this way can be very miss leading. Sometimes there will be a drain indicated until all the circuits stabalize. Some times they need full battery power to do this which they can't get thrugh a volt meter. An amp meter is the correct instrument to use but most multi-meters don't do the job. You must have a meter that can read down to 50 miliamp but be able to handle up to 10 amps if needed. Most multi-meters can do one or the other but not both at the same time.
But, if you could tell us what your problem is maybe we can help. For instance, i assume that your battery goes dead. How long does it take? Overnight? 2 days? etc. Could it be a charging problem. Does it have an aftermarket radio in it.
Give us some clues and we will try to help. By the way, this is the type problems I work on every day.

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84 500 SEL (307,xxx miles)