[Misdirection]

I dont think what your experiencing is normal. I suspect that your experiencing voltage loss somewhere between your battery bank and the solenoid or starter. The first thing I would do is get a volt meter and measure voltage at the starter while cranking on one bank, and then with the parallel switch on. That will tell you alot. Then start checking cable connections and cleaning them and checking the crimping. Also check the ground at the engine.

Report back what you find.

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