Possible MH Engine Battery with Amp Hours Too? UPDATE

You "could" replace the little battery with a Concorde Lifeline. Without over-discharging it, it would serve as a dual purpose battery for light loads like lighting. But you would need to keep an eye on voltage to stop the draw before the battery goes dead flat. The lifeline is inherently deep-cycle and not a dual purpose marine battery as it has a hefty cranking ability and far better tolerance than any RV battery. They aren't cheap.