The meter likely is simply showing the instantaneous charge state of the battery (converted into a row of lights rather than a number).
When the battery hasn't been used for a while and there is no load or charging source, the voltage roughly corresponds to how well charged the battery is. It's a cheap and easy solution for them to add but requires a little understanding to use in a meaninful manner. When you apply loads the voltage will drop almost instantaneously (depending on the load), so even testing with a meter at the posts doesn't work.
If you turn off the lights and it returns to a "full" charge state, the battery is likely still full or close to it.
If you apply a heavy load for a short period of time, it may show up as heavily discharged even after the load is removed but the chemistry of the battery will catch up and the charge state shown will gradually come back up over 10-30 minutes (assuming it isn't actually depleated).
A quick check found a vent fan pulls around 36watts. Assuming 6 15watt bulbs, the bulbs were pulling 90 watts or about 3 times as much, so that is likely why you see a difference in the meter.
An easier solution is to get the amp-hrs for your battery and calculate how much time you have. Divide the watts by 12 to get a rough idea of the amps each item draws and then multiply how long you expect to have it on to get an idea of how many amp-hrs of battery you need. Only assume that you can use 30-50% of the available amp-hrs.
If you do persist with the run it till it dies test, try to stop before reaching 50% discharge. Deep discharges are hard on batteries, hence the plan to only use 30-50% of the available amp-hrs.
