Glacier Bay Fridge/Freeze overdraws amps when trying to start

[Liefo]

Hi folks - New to the forum, and to the cruising world. I bought a Hunter Passage 42 in October and am now living aboard in the SF Bay Area.

We have a Glacier Bay Fridge and Freezer unit from '98 that was working like a charm up until a few days. This appears to be a pretty sophisticated high-octane system overall, and definitely more complex than my amateur electrician skills may be up to. That being said, I've done everything I can to troubleshoot the issue and a repair guy isn't available for a week, so I'm hoping someone on here can recommend some additional things to try out myself.

Here's what I know:
-When I close the breaker which pulls directly from the battery, so long as the fridge and freezer switches are in the "on" position the system attempts to start up, including clicking at the starter solenoid on the fridge panel, but all other 12V systems die momentarily (stereo turns off, lights dim), so it seems to me as if it is overdrawing amps. The system fails to start up and continues to retry every 5 seconds or so until I open the breaker.
-If I close the breaker and leave all switches off, I'm able to see that I'm getting 14.61 volts entering the fridge electrical panel. There is a "manual" startup switch that I can flip which causes the compressor motor to run (direct drive, no belts). When this happens, I am measuring only 6.5 volts at the primary lead of the starter solenoid (the hot lead of the two smaller leads).
-My first assumption was that I had a bad starter solenoid, so I replaced it. No luck, good bye 60 bucks.
-My limited knowledge suggests that some fault elsewhere in the system is causing this voltage drop at the solenoid, but I'm really unclear what is the next most likely culprit. Corroded wires, a faulty switch somewhere else in the circuit, something specifically part of the starter (is the starter inside the casing of the motor?) - something else entirely?
-I've read that this could be corroded wires, though I've checked all visible connections at the panel and they look fine. This evening I will check the connections to the thermostats inside the fridge and freeze compartments - is it possible that corrosion or bad connections/improper grounding of the thermostats could cause the voltage drop in the entire circuit? It surprises me that a manual switch which simply runs the compressor motor would suffer from issues at the thermostats.

Why might the voltage to the solenoid drop to 6.5 when the system tries to run? Why would this problem still occur when the manual switch is flipped which only runs the motor - does that help isolate the problem at all? Any recommendations on the next things to test to isolate where the issue could be?

Is it possible that this is a battery issue unrelated to the fridge? I have yet to dive into understanding the setup of our battery bank, which will be completely uncharted waters, but we supposedly have 4 electrical forklift batteries designed for 80% discharge with total capacity of 800AH. Not having other battery issues.

I have a huge book of manual/technical pages about the system, many of which I only vaguely understand. Some of the manual pages, including a wiring diagram: https://www.dropbox.com/sh/wggxb509f...3jiOqPt_a?dl=0

Thanks so much in advance.

-Lief