I'm just guessing ... perhaps there is a problem with the ignition switch. Does it have a wire feeding into it that is always hot?
My wife's turbodiesel years ago developed an electrical drain that would drain the battery every 10 to 14 days. I removed half the fuses, no change. Half the remaining, no change; all, no change. I bought a solar battery charger that plugs into the cigar lighter. That fixed it.