I would leave the starting battery back near the motor. I have one deep cycle battery on a switch and it powers everything but the motor. I have a cranking battery hooked up straight to the motor, no switch. I have not had a problem this way and it has worked the best for me. I just replaced my cranking battery a few weeks ago. Not because I was having problems, but I got to looking at it and saw that it was 8 years old! I replaced with new one and I'm now using the old battery in my camper. A friend of mine has his batteries hooked up together on a switch and it drives me nuts every time he starts his engine and spikes the GPS.
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