To measure total battery drain -- very simple:

Remove the negative cable from the battery....
[this can also be done by removing the positive cable, but it's considered safer to always remove the negative cable first, and reconnect the negative cable last.]
...and then hook your VOM's leads to that negative cable and the negative post of the battery. In this way, everything possibly powered by the battery must (without exception) pass through your VOM.
But take these precautions:
1) be sure to observe proper polarity (red wire to the negative cable, and black wire to the negative battery post) if you have an old-fashioned analog VOM.
2) be sure to have the VOM set to the appropriate scale (start with the largest scale -- even the special 10 amps scale if you're concerned about a major drain); you can also set progressively smaller scales if your measured amounts indicate that it's safe.

If you want to narrow down your search (for a major drain) from there, you can start by measuring the amps across your fuses (remove a fuse and put your VOM's wires across the fuse contacts).

Good luck.