Severe discharge of 12 v battery
Your batteries are so called Lead acid batteries and if you keep them long uncharged below 40% you come into the danger zone. Below 20% charge you can loose capacity.

Here is why:

During the discharge the active mass of the battery changes from lead dioxide and spongy lead into lead sulfate. This lead sulfate has a fine structure and returns during the charge back to lead dioxide and spongy lead.

If now during a deep discharge the recharge does not take place in time (to late) the lead sulfate starts to crystallize thereby forming large crystals. These crystals block the pories of the active mass and form a barrier over the plate surface making it impossible to charge the battery. The result is sulfatation and permanent loss of capacity.

Therefore if the batteries did not remain to long in an uncharged situation the damage could be small to nothing. If it it took a long time before recharging you could end up with permanent damage to the battery. You might have been lucky.