12V circuit only works on shore power or generator

Several years ago I had a battery that charged up and would deliver 12.5 volts but deliver current for only a couple minutes to my trailer lights before things went black. I pulled it off the trailer when it was supposedly fully charged and took it to an Auto Zone where their tester showed it was toast. A new battery solved the problem. Charge your battery fully and then take it in to be tested. My guess is you have the same issue I did.