That is not a diagnosis, but it certainly gives you a clue towards a "most likely" cause, and thus can begin a systematic and logical progression to nail down the culprit. If it's a hydrolock (or other mechanical interference) then you have that PLUS an electrical problem.
The starter can't draw enough (without starting a fire or at least smelling badly burned) to pull the batteries down that far. It would have to be melted solid inside. Good batteries, fully charged at that, you should indeed see the needle twitch, but not that much. That would indicate that the problem is more than likely still an electrical one, not mechanical.
Put your volt meter across both batteries. Black lead on the low negative, red lead on the high positive. Set the meter inside the passenger door, and press the starter button. Some drop is expected. six to ten volts dropped MAY indicate a bad starter, MAY indicate a stuck motor. More than that dropped indicates junk batteries. Little to no drop indicates a bad starter or bad wiring...
If that passes, Load test the power circuit. Red probe at the high battery positive, black lead at the starter solenoid top post.
Load test the ground circuit. Red probe at the starter ground stud, black probe at the low battery negative post. Perhaps do that before the power side, since from your post I gather there are unchecked connections in this "leg" of the circuit. That would (could) also effectively shunt all of the in-cab accessories.
I wouldn't want more than a volt on either the ground side or the power side while it was cranking. Higher than that indicates resistance in the circuit. You need volts, because your ohm setting is very low powered, with only a few miliamps which the circuit can obviously support, and will not yield a usable result on a circuit with this large of a current draw.
When testing voltage drops, clamp your clamp carefully. On the starter it would be directly on the metal stud, and NOT on the wire terminals themselves. At the battery, the probe should contact the battery post it's self, not the terminals or adapters. This makes the test all inclusive of the wiring/circuit related items for ruling out batteries and starters. You can use the tests to break down the failure further, to dirty terminals, rusted corroded wires inside the crimps, open wires inside the insulation, etc. This will get you started.
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