Possible that one of the batteries has a shorted cell. When plugged in( converter charging batteries) the voltage your panel sees is not actually the battery voltage, but the voltage delivered by the converter. Once you unplug and and the converter is no longer charging batteries, you see the battery voltage. The initial voltage you see immediately after removing the charge voltage is the surface charge and not a true indication of the battery condition. You need to let the batteries rest for about 24 hrs for a true reading. That reading will be lower than the initial one. As mentioned I would remove the batteries and have them checked out, if you have a bulb hygrometer, check each cell yourself. If you have a bad battery, you should replace both as batteries used in banks should always be the same or very close in capacity, type and age.
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2015 Freedom Express 248RBS
TV 2015 Silverado HD2500 Duramax
TST Tire Monitors
Honda 2000I + Companion
2 100W solar panels
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