Unplug or pull fuses on everything connected and then put fuses back one at a time and see if voltage drops then plug up connections and see if voltage drops. I don’t know of a simple or easy way to find something like this. By pulling fuses and unplugging connections at least you hopefully can narrow it down and then start looking at what is connected to further trouble shoot. I have a dedicated accessory battery just to minimize the risk of a dead cranking battery. I would think that any short should shoot a fuse or trip a circuit breaker.