Take out starter battery and go and get it load tested.
Is the house battery involved in starting ? Eg supplies power to solenoid or glow plugs ? If so, get it tested also.
Got to make sure batteries are good before messing about looking for failed connections.
I had a problem with corroded wires. They were only copper, not tinned. Found by using a current detector.
Have starter battery on shore power charger
while starting. Undo starter connection and use a friends battery to jump. All the above will help isolate the problem. Could be starter itself, connections, anything. Start at batteries and work up to starter
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