have you measured the draw on the batt? are you sure there is one? i used to hook up an amp meter to the battery in series and pull fuses until the draw goes away 1 at a time, that will locate the source then start testing. if you have an amp clamp it's even easier and safer. alternators can draw batteries down if the z-connector (i think that's what it is called, maybe y) is bad because it's where the a/c voltage is sent to ground. if it goes bad, i can start drawing the dc voltage to ground also and we'll dump a batt pretty quickly.
i haven't done this in about 5 years, but that is what i remember, i could be wrong.