I assume you mean the battery is being drawn down overnight, or over a week or something.
A quick way to see if it's your aux fuse block in the first place would be to hook your multimeter inline between the positive terminal and the main cable to the fuse block. Should be no amperage draw when everything is shut off. If your aux panel is drawing power, then pull fuses and check for draw on the individual circuits.
Could also be a battery on its way out. How old is that Sucker?