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Hi my name is Mike,
this could be as simple as a loose battery cable connection... If a ground or positive cable has a loose or corroded connection, it will not pass the amps required to run the starter motor.
Check the terminal connections at the batteries, back of battery switches, the ground on the engine block, and the Positive where it connects to the starter.. All connections must be tight. If you have wing nuts on your battery terminals, carefully tighten them wit ha wrench or pliers as tightening them by hand is not sufficient.
Try checking these things first and get back to me with your finding s and we'll take it from there.