It sounds to me as though something is draining the battery over time. It's a short and it's like something being on when it is supposed to be off.
Test for a drain on the battery by a using a 12-volt test light or voltmeter. Take a full reading across the battery posts to get total battery voltage. Write that down. Then disconnect the negative battery terminal and hook the 12-volt test or voltmeter with the positive test lead on the disconnected negative battery cable and negative test lead on negative battery post. Turn everything off, close the doors and if you have a hood light remove the bulb. With the negative battery terminal disconnected and a test light or voltmeter between the cable and post see if you have a drain showing on your test light or voltmeter. A light on the 12-volt test light or voltage reading on the voltmeter indicates that something is draining the battery. If so start by unhooking the small wires at the alternator and see if the test light goes out or dimmer or if the voltmeter drops in voltage, hopefully to zero. If the 12-volt test light goes out or the voltmeter now reads zero or less than 0.2 volts then you have found the short. If the voltage is not zero on voltmeter or test light does not go out, the short is somewhere else. If the voltmeter just drops some or the 12-volt test light just dims a little you still have a you have a drain there but still a drain somewhere else so start pulling the fuses 1 at a time and when you do find out when you pull a fuse and the drain gets lower then put the fuse back in hook up the battery and make sure everything on that circuit works. Let me know which fuse you pulled and I’ll upload the wiring diagram so you can check everything on that circuit and make sure that it works. If something doesn’t work on the circuit that is causing the drain then it may be short-circuited with another circuit of shorted to ground. That may help find where the short is if something don't work. So with your 12-volt test light or voltmeter test the wiring to whatever doesn’t work to find out where it has voltage and all of a sudden looses voltage and then repair that spot. If any drop some or allot write it down which fuse it was and if using a voltmeter write down how much volts it dropped. Remember if you have to keep the door open to pull the fuses just push in on the door switch after every time you pull a fuse and go to look at the meter. When all fuses are pulled and the alternator wires are off you should have no drain and written down everything that was causing a drain. If you still have a drain with every fuse pulled and the alternator wires off, then it must be something that is unfused such as anything that may be fed from the light switch.Last if you have all the fuses out and still have a drain disconnect the big wire at the alternator. If the drain now goes away you have a bad diode in alternator.
When you find out which fuse it is and what circuit or circuits are causing the drain, let me know and I'll upload the wiring diagram.
When you hooked up the 12 volt test light or voltmeter did it show a drain? If so was you using a voltmeter or test light? When you pulled fuses did you leave them out untill you had them all pulled? Did you pull all the fuses out of all the fuse boxes,blocks?
There are 3 fuse blocks.
Sorry, I was napping.
If the voltmeter wasn't showing a drain, there was no use in pulling all the fuses. Are you sure you hooked up the voltmeter correctly? The negative test lead toward the negative battery terminal. What scale did you have the voltmeter on?