Battery Options for 1961 Land Yacht

I recently bought a semi-restored 1961 Land Yacht. It did not have the DC system operational. I have a thin box under the front window for the battery(s). The previous owner said it had 2 6v batteries tied together for 12v.

Does anyone know what kind of battery(s) I need? I would assume that 1 12v battery would be best but don't know if they even make one that thin.



Thanks