Also once a battery has been discharged below the 50% charge state without being recharged right away it is in most cases toast after that.
Not holding their charge very long like they used to is the first clue.
Batteries don't grow on trees for us... we lost our first battery just months after purchasing our trailer in 2008 from being over charged by the single mode converter/charger and all the battery fluids were boiled out.
After that happened we changed out the converter/charger for the smart mode converter/charger being higher amps rated that is known to always work when you expect it to (PD9260C), added larger battery cables everywhere using three four position (OFF-BAT1-BOTH-BAT2) heavy duty rotary Blue Sea battery switches, changed out all high current automotive lamps inside the trailer with LED lights, and added more battery capacity. Now we can select any, all, or none of the batteries being used. A couple clicks of the master four position battery switch completely disconnects the batteries from the trailer 12VDc system. I will switch 'OUT' my batteries when not being used for a long period.
During the winter months I will charge my batteries maybe a couple of times using the on-board converter/charger by connecting to the garage 20AMPS service using an HD extension cord or run my 2KW generator that needs to be run under load anyway...
My remaining batteries installed in 2008 are still in use today and are just now starting to fall off on performance. One of three batteries is really doing worse than the other two batteries on holding its performance.
I still can draw some 22AMPS from the batteries when camping off the power grid from 6PM to 10-11PM at night and only drop my batteries to around 12.0VDC by 8AM the next morning. This is when we initiate charging of the batteries back up to their 90% charge state by connecting the shore power cable to our 2KW generator using a RV30A-15A adapter and allowing the on-board smart mode converter/charger to recharge the battery bank in a three hour generator run time.
Been doing this routine since 2009 camping off the power grid in our OFF-ROAD POPUP camper.
I have a central located three meter BATTERY MONITOR panel inside the camper that monitors the 12VDC level of each bank as well as how much DC Current is being drained or put back into the batteries when being charged.
The OEM BATTERY POWER MONITOR meter shown here is worthless to me..
The Radio Shack Rocker switch beside the JENSEN AM/FM Radio is what we use after setting up camp to operate the radio ON-OFF and not lose our radio sittings. This is equivalent to the car ignition switch if the Jensen radio was installed in the car. If we need the radio to be totally disconnected then we turn it off using the built-in ON-OFF switch on the radio panel. No parasitic current is being drawn from the radio with the built-in front panel switch operated to OFF.
I also do routine maintenance on the batteries usually every month checking on the battery terminals for corrosion, checking fluid levels, and looking for abnormal signs around the battery cases.
My parasitic drain is around 1 AMP with every thing obvious turned off inside the trailer...
A little bit of effort pays off big in the end being around your batteries.
Roy Ken