I removed fuses and let mine sit overnight to localize which sub system was drawing down the battery. I got lucky and found that my problem was the internal (courtesy) lights, on the third try.
I've seen the courtesy light stay on in the glove box with the door closed. The door switches make a slight contact, not really enough to turn on the lights, but enough to drain the system. Audio amps that ran 24/7. The battery itself covered in acid and mud drain through the dirt. Various water soaked connectors drain the system down. And even current flow backwards through the alternator to ground. The only way to test a battery and be fairly certain it is good is with a load test and a specific gravity tester. Sometimes the battery is good, but the electrolyte is tired.
Check the obvious first, like anything that normally has power with the key off.
A good amp meter may be helpful, but they can be expensive. A volt meter less so. There are often very small current usages in the system even with everything off, that don't really mean anything. They measure 12-14 volts but draw so little amperage as to be irrelevant. A volt meter is less useful in chasing down current draw and line leaks.
If you do a lot of mudding, you have to clean your rig up good. Little line leaks from mud/water seeping into connectors adds up. Wet mud really doesn't conduct well, but a little here and a little there adds up. Salt or coolant mixed in the the mud, conducts pretty good and can drain your battery fast.
