start pulling fuses until that 20 amp fuse stops blowing. but i'm pretty sure that wire is main power that goes through the ammeter to fusebox? im sure @Mattax will correct me and provide more insight into possible issues
Yes. It is the battery feed to the main circuits. Removing fuses could help narrow it down but since all of the fuses are 20 amps or less, they would have blown.
It seems the situation is thus.
There is a short in one of the power feeds or main circuits. It drained the battery but did not melt a wire or connection, yet.
If you look at the ammeter and saw no discharge, then the short was before the meter.
At this point, I think the safer method to narrow down the location is to leave teh battery disconnected and use a multimeter (resistance or continuity) or self powered test light.
Disconnect the alternator feed at the bulkead. Test that. Then confirm its inside by testing the terminal.
