Battery draining can be as simple as one draining into another if they are wired in parallel and are equalizing to the battery of the lowest capacity. Otherwise, get a multimeter (one with a 10amp setting preferably), take off the positive battery cable and put the voltmeter between the + post and + cable, any voltage with everything off will give you a reading. Pull fuses one at a time until you get a zero reading, poof, there's you culprit. Then isolate that circuit to see where it's grounding out or using power. Some boat radios, AM/FM and VHF will draw just to keep the radio station memory alive.