Battery going dead

[bignoisey]

I have a '99 3500 Ram van RV conversion. It has a dual electrical system with battery isolator between primary and RV batteries. After the vehicle sits parked for over about three or four days, the primary battery is dead. I've carefully checked that everything (lights etc) are turned off, but the battery still dies. Fortunately, I can just jumper across the battery isolator to get it running again, but that's a nusiance - especially when I'm in a hurry. Before I changed it, In the morning I noticed the battery was weak sometimes so I replaced it even though it was only a couple years old. Since I replaced the battery we've been using the van almost daily so I thought the problem was gone. Nope, its not.
I sort of remember reading about this problem in either this or some other forum. Can anyone help me?
Something must be leaking current even though I think its turned off. I wish cars still had ammeters so I could just start pulling fuses and see which one stops the current flow.

[fredjacksonsan]

There has to be a little draw from somewhere. Isn't there a way you could hook up a voltage meter, and then pull each fuse in turn?

[fredjacksonsan]

Or maybe use the voltmeter across where the fuse was to check the draw.