I didn't know these things had a 12 V battery until after I ordered it. Oh well. It will be a maintenance item that I did not plan on. Based on your post I did a little research. The link below was quite helpful, and although it is intended for solar power applications it is applicable to some degree.

Deep Cycle Battery FAQ

From another source it appears the OEM battery is a gel type. That explains why they limit charge current to 5 amps. Over voltage is bad for a gel type battery. The reason for using a gel is that it does not gas. The hydrogen and oxygen produced are reabsorbed within the battery, so it can be sealed.

Based on the link above, it appears AGM or Absorbed Glass Mat batteries offer the same advantages as gel, but without the overcharge sensitivity issue. They are expensive, but do not gas either, so no need for venting. The Optima Yellow and Blue batteries are of the AGM type, and you can find other marine and RV deep cycle batteries of the AGM type as well. The ideal battery would likely be a pure deep cycle AGM type. However, on a quick look I did not see any. They would be more likely for a RV trailer application as they never see any cranking service. But, it could also be specialized and expensive.

Of the Optima batteries, I would suggest the Blue would be the best as it is intended for marine service, with some models designed for deep cycling. Of them if it would fit (10" long), I would suggest the Blue D34M.
It has about 20% more reserve than the Yellow D35, and is more intended for deep cycle with potentially 50% more recharge cycle life. Not cheap though!

Hope that helps some,