[Blas]

Your first move would be to check the battery out. I would pull it out and have it load tested, which is ususlly a free service at a parts store. Be sure to clearly mark the positive and negative wires. I recommend that if the + side wire is not red, put red tape on the wire. Sometimes, when battery cut of switches were added to the + side, they came with a section of black wire. An accident waiting to happen, if not for you then for the next owner.
If the battery checks out ok:
Easiest thing to try, which also costs nothing but time, is to pull fuses - and try to isolated the circuit the power drain is on. After you have done that, then do the detective work. Start by pulling half of the fuses. Keep us posted. I'll bet it's a bad cell in the battery...