“When I hook up the posi and jump my meter from the neg term to the neg cable, I get -12.76.”
Not exactly sure what you’re doing here, but if you have both battery terminals connected, you should measure around 12 volts from the pos terminal to any metal ground on the vehicle and 0 volts from the neg terminal to any metal ground point. If your battery is going down due to standby current, you’ll need an ammeter. But check the obvious first, such as courtesy lights that aren’t going off, etc.
I think his volt meter is in series from neg terminal to neg wire and he’s finding a drain of unknown amperage...not enough to blow a fuse, evidently. Seems like pulling fuses would isolate it to a circuit.