For me it was dead starter batteries combined with a Battery Separator that was always on, allowing the dead starter batts to drag the house system down.
So first I'd disconnect the Separator which isolates your starter batteries from your house. I seriously doubt *that* is the problem but it's the first step in isolating your starter side.
Second turn everything you think is on the starter batts off.
Then disconnect the negative (on a dual-batt under the door diesel the only negative is grounded to the frame towards the rear from the batts).
Brush the negative terminal across the frame and if you get spark, you have draw. You can disconnect things one after another and try to isolate the problem.
It's a bit brutish but apparently it works. I've read one persons experience (here? email? yahoo groups?
) where they had a drain inside the alternator and found it using this method.