On our boat we didn't have an analog battery gauge. We installed a cigarette lighter volt meter to make sure the batteries were being charged. At the same time while anchors down with the radio on, etc. we never drain the batteries to the point it wouldn't start the engine.
One of these:
Now, for the engine's battery you can just look at the dash gauge, but it would be nice to have something a bit more sophisticated than the battery dummy lights on the control panel. Maybe something as simple as an extra LED that'll let you know the batteries are being charged. Even better would be a display similar to the cigarette lighter meter that tells you the exact voltage.
This would be a cool mod:
Tap into the battery wires that feed the dummy lights and install the digital gauge on the control panel. Hmmm.