Get a 12 volt 60 watt light bulb with a pig tail socket, connect it to the battery first and have someone energize the bow thruster or
winch. If the light dims a lot, it's probably the
batteries or battery terminals. If the bulb stays fairly bright, move down the
electrical line and hook the bulb near the mid point or pilot house panel and energize the bow thruster again. This is an easy way without proper meters. I would do this without expensive
electronics on. I've seen
cables with a pin hole that caused the wire to be corroded in the middle of a good looking cable causing very low voltage at the
motor. Low voltage will be hard on the
winch or bow thruster
motor. Another way is to put the negative terminal of a digital meter on battery negative with a long wire without the
charger on. Load up the main panel with everything on that is possible, at least 25 amps, check the voltage at the battery, then move along with the meter and follow the positive wire and see if the voltage drops while keeping the load the same. Then swap the long wire to the meter positive and do the same following the negative cable with it loaded. Every
electrical connection may drop 1/10 th of a volt, that's normal. Good luck hunting.