I thought I had heard or read of someone with a coach like mine with the slideout battery box behind the tire on the passenger side. Actually, move the engine battery up infront of the engine and add an additional coach battery in the slideout. Did I dream this or did someone actually do it??

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Shortcuts Always Take Longer

I have been considering that, too. Or putting a sealed chassis battery in a storage bin. Or a regular battery in a sealed, vented battery box. I saw one in a catalog. Milodon or Moroso, I think.
But we keep getting better at conserving battery power at night, so maybe not. It is best to have your deep cycle batteries as closely matched as possible. Otherwise, you need to charge each bank seperately.