Paralleling marine type battery with a pair of 6V GC batteries

[sundancer 87]

You won't gain much with the 'marine' type battery. It has a mcca or cca rating, meaning it isn't a true deep cycle battery. If you are looking for a longer cycle time from your batteries, which the deep cycles have, the OEM battery won't do it. You can leave it out and probably be better off without it in the battery bank.