I agree the indicator panel is useless.. A 921 is what about 18 watts. 1.5 amps. Times six is 9 amps per hour
your Group 24 has what, 75 amp hours, (C/20 rate) and you can only use about 37 of them, And when you draw faster (the 20 hour rate is about 3 amps or two light bulbs) when you draw faster the batteries go down faster still, I do not have the chart handy but figure you get about 3 hours lamp time on that battery with six 921's.. This of course does not include the fan and other things that draw power.
Your panel indicator is not measuring battery voltage either, there is voltage drop in the wires between the battery and the fuse panel and that's where it is measuring at.. plus the voltage to state of charge chart is only valid for STATIC (At rest) voltages not for operating voltages.
On and that "You can only use half" applies to DEEP CYCLE, on Marine/Deep cycle, you should not use that much.
I suggest additional battery of if possible upgrade to a pair of GC-2 golf car six volt in series forming one big 12 volt battery (230 amp hour, roughly a 4-D)
