My 2 zincs:
1) The battery panel is not particularly useful to determine battery status while dry camping.
The top light (charging) will go out almost immediately after any load drops the "charger induced" surface voltage below approximately 12.7 volts (100% charged).
The "GOOD" light (and the other two below it) will stay on until the battery voltage drops to approximately 12.1 volts (50% remaining) at which point the battery should be recharged in order to preserve the maximum amount pf recharge cycles. If you walk by and notice only 2 lights on, you will most likely have reduced the useful life of the battery by the time you notice because unless you caught it JUST when the G light went out, you won't know the actual level of depletion.
BECAUSE the second to the bottom light is "FAIR" and will stay on until the battery voltage drops to approximately 11.6 volts (20% capacity remaining) which is dangerously low and permanent capacity reducing damage is occurring.
If you miss that one and see just the bottom light "LOW" lit you have waited too long and your battery is most likely dead (10.5 volts by definition). That light will remain illuminated until the battery voltage reaches 6 volts. That LED will NEVER go out unless the battery is removed or severely damaged internally (like frozen or shorted internally).
2) Only a battery management system like those mentioned (I use the Trimetric TM-2025RV) can give you an accurate battery voltage AND percent capacity remaining at a glance.
Not only that but it can actually display the Charge and Discharge CURRENT going into (amperage reads "minus") and out of (amperage reads positive) the battery(s). This will allow you to monitor the time to charge (so it is convenient to you) and the amount of generator run time before quiet hours you will need to get the battery to the point it will last through the night with the furnace running.
The Trimetric will also monitor the last time your batteries were equalized (in multi battery systems) and can monitor two battery(s) or banks. If you have solar or an external generator you can wire it so the B2 lead monitors that source instead of a second battery.
It is really cheap and easy to install. TriMetric Model Descriptions, Present and Past | Bogart Engineering
The newer version (the 2030-RV) can also sound an alarm at a level you set so you won't be surprised by a lower percentage than you want when next check the display.