Those "charge level" lights aren't of much value, since you have no idea what voltage level each indicates and they probably aren't all that accurate to begin with. But 1/3 charge would be pretty low on any scale and I'm not surprised your Co detecor beeps - they are widely known to be voltage sensitive and will often beep long before the lights dim and other symproms show up. The LPG alarm may be sesnitive also.
You can buy a digital gauge that shows the actual voltage - I have one that plugs into a standard 12v outllet and it was under $15. My coach doesn't have a direct voltage read-out either, so I wired an outlet direct to the battery just for this plug-in gauge and placed it where I could easily see it, so I can always read the true voltage right at the battery.
You didn't mention how many batteries you have or what size & type, but the furnace is a notorious consumer of amp-hours. The fridge is not and does not, I believe, draw 4 amps when in LPG mode, where only the circuit board is powered. That 4A fridge fuse you referenced is probably for AC power to the heater and not powered when in LPG mode. A videotape player (using an inverter? DC powered?) is probably not a large consumer either, but any use that lasts 90-140 minutes of continuous operation will have a noticeable effect. Then there are the 12V lights and such as well.
As a rule of thumb, we often estimate that you have about half the rated amp-hours available for actual use.