Do you have anything that is wired directly to the battery that could be drawing power (ex. the winch could have a faulty switch which would not activate the winch but draw a minute amount of current) ? How long does it take to kill the battery (a day, a week) ?
Have you thought of installing a switch that would cutoff the battery power directly at the battery ? If that doesn't solve your problem, then you know that the battery is the problem.
2006 Blue Grizzly 660, hand & thumb warmers, Warn 2.5 winch, WES Rear passenger seat/storage area with hand warmers, Quadrax 54" snow plow, Garmin 60CSx GPS, ITP 25" Mudlites