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[Larry S.]

I'm not sure what your reasoning is for doing what you are proposing. I was suggesting a relay that would connect the deep cycle battery in parallel with the starting batteries whenever the engine is running. That's how the trailer towing package is set up on my truck. If for some reason you want to keep your computer and other accessories alive when cranking or other times you may have the starting batteries discharged very low, I could see that condition if you have an electric winch operating from the starting batteries, all that would be needed would be a manual switch to open the relay and disconnect the deep cycle under those conditions. Then when the engine is running and the starting batteries are sufficiently charged energize the relay and your truck will be using all the batteries. If using a diode isolator you may have an issue with maintaining full charge on the deep cycle if all accessories are powered from it and in use, but in most cases there should be no issue.