The low battery will sap power from the other battery in the bank causing a net loss of useful power since you cannot isolate a single 6 volt battery. My guess would be there is a short in one cell and it is POSSIBLE that it could be saved by discharging and emptying acid/electrolyte out by turning it upside down while discharged. You'd have to add battery acid and fresh distilled water and recharge the battery and see what happens but may not be worth it for a one year battery. NO guarantee it would work and not saving that much either. You could CONTINUE to use it even with low voltage if the power loss if acceptable (camping with electric hook ups nearly always) but watch it to be sure the case is not heating up from the short (if there is one) because they can be dangerous if they overheat. If it were me I'd replace it.
Good luck / Skip