I have a direct short which drains both batteries in less than 1h
Remove positive cable from battery & connect a 12 volt test light between battery terminal & battery post. Anytime light is on, you have a draw on battery. Don't forget your stereo will cause a small draw all the time. Light will dictate how much draw, if it's bright, lots, if dim, not too much. If it' bright, and you pull a fuse & it dims down, or goes out, you've found circuit with fair bit of draw. If worse comes to worse, I always find it easier to make sure I know where they all go back to, then pull them all & start putting back one at a time, light will tell you where you have a draw, mark it down, pull it back out & try another, until you found all sources of draw, and recorded the brightest one(s) Now go from there with info you have. If you put one in & no light, you can leave it in. Good luck, at least it will isolate the culprit(s).