One of the other options is to switch high-capacity house-battery type Group 24s on a separate circuit in the battery box and add a smaller starting-only AGM battery such as a Group 31 on a new circuit. The advantage of the AGMs is that they have very lax angle tolerances for mounting and can even be mounted sideways if need be. The Yanmar engines that J/30s use don't have terribly high cold-cranking amp needs and can definitely be started on a smaller battery. Another good option to consider is reducing your draw by converting all of the interior light fixtures to LED if you haven't already, that was some of the best money that I spent on Mondial. The insanely over-engineered spring-loaded bayonet-type halogens that come standard suck down power at something like 5-10 Amps apiece.