That is a pretty long run for putting in a 3rd battery. I understand the desire to have a battery near the bed, though.
It would probably be easier to put the 3rd battery in the passenger front outside storage and simply run the wires for the 12V outlet to the rear (that way the jumpers between the batteries can all be 3' or less instead of 15'!).
I think, overall, though that Dave's suggestion may be easier...
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