Batteries/Battery Charger-yet another question

G'day all,
Recently on a trip to Alice Springs we had several days of overcast clouds and therefore limited solar imput to the point that we had to connect to 240 volts.
After this we have noted that during the day with good solar imput the batteries charges is up to 14.2 volts. With normal use - fridge, lights, TV - the volts will drop to 13.3 at night.
In the morning the volts will be down to 12.2 but when we activate a LED light the volts will drop immediately to 11.0 or lower on some occasions (10.8) and the load will immediately go to "OFF". The panel will the scroll through the system and 12.2 or more will be illuminated for a brief time prior to dropping voltage as above and going to "OFF". This sequence will continue until there is sufficient solar imput and then system operates normally. The batteries are approximately five years old.
Could this problem be the batteries, the battery charger not operating correctly or some other cause.
If it is the batteries, I note that there have been several posts. What is the best brand/capacities/etc for the BT - 4 x solar panels, 4 x batteries- and does anyone know of a supplier in Adelaide.
With Thanks. Greg