As already mentioned those 25% increment LED lights found on most RV panels are nearly useless, A digital voltage monitor like this is a big step up https://smile.amazon.com/LinkStyle-Digital-Voltmeter-Waterproof-Motorcycle/dp/B0787QFQ5N though even this does little to tell you the condition of your battery, if you want to have a good idea of how much power you actually have available in your battery bank you need something like this Victron battery charge monitor https://smile.amazon.com/Victron-Energy-Battery-Monitor-BMV-700/dp/B01BVQR0V8/ which actively tracks power going into and out of the battery bank.
Now for the simplified version, batteries loose capacity as they age, this partly depends on how you care for them, if you drain the fully, they may only last a handful of discharge cycles, if you drain them half way they will typically last about 500 charge/discharge cycles, if you drain them only a little bit they may last 5,000. Abuse like deeply discharging and not maintaining battery water level (using distilled water) will shorten their life.
Picture a battery as like a gallon jug which holds electricity, how much it holds depends on its condition, over time it looses capacity, sort of like filling a gallon jug up with gravel, the more gravel you add the less room there is for water, so soon that gallon jug may only hold half a gallon of water, and soon after that it may only hold a quart. There are also things that can kill a battery nearly instantly, like a shorted cell, the voltage may still look ok, but it nearly instantly looses its capacity when you try to discharge it.
If the water level is low in the batteries you can try adding more, but this is sometimes at best a temporary measure, batteries normally loose a little water under normal operation, and should be checked and topped off every month or two. Note only fully fill batteries when they are charged, as the water expands with the charge, if you fill a discharged battery it will overflow acid/water once it is fully charged. If the water is very low then partly fill it, charge, and top off to the grid plates once the battery is fully charged. There are you tube videos that show how to do this, note don't let battery acid get on your skin, and wearing safety glasses is a good idea.
p.s. the type of battery currently installed in your RV may not be the best option for boondocking, before you buy new ones consult us on what type you should buy. Many RV's have the cheaper lower capacity dual use batteries in them, not true deep cycles
