I would consider pulling each fuse and taking a reading with a meter to see which circuit is drawing power when everything is shut down. You may have to disconnect some lights or other accessories but it is the one line at a time process of elimination. It's not easy and it takes some time but the more you learn the more likely it is you will isolate the power drain. The starter, battery and solenoid are the easy finds. The rest could be almost any electrical switch,motor,light or sensor in the vehicle. Good luck as sometimes it is luck which leads to a fast resolution.

Bruce