C - Charging s/b 13.2 volts or above and indicating a charge going into the battery. 13.2 to 13.8 will be real slow. 14.2 to 14.8 will be much faster.
G - Good s/b 60% or better charged. Use the lights and furnace sparingly.
F - Fair is closer to 40% to 60% and time to use even more conservation and charge if convenient.
L - Low, time to restrict all 12v use and immediately recharge the battery.
Or many indicators prove to vary quite a bit. Get a small voltmeter to verify using the above charts.
BTW you should always have a battery connected when towing.
