I found a similar situation last season, it was the wingnut on the positive terminal of the starting battery. It somehow worked its way loose. I also found corrosion on my battery terminals which is unusual, so everything gets a coat of dieletric grease now when they get assembled at the start of the season. I would be curious about a couple of things though, first is the water level in that starting battery, second is the actual voltage. I have had two different brands of lead acid marine starting batteries fail suddenly. They would no longer hold a charge. In the process of the charging systems trying to bring them up, the water cooked out in pretty short order.

The good news is that you seem to have a pretty stout house battery setup as a fallback.