The drop from 13.4 to 12.9 very quickly is just fine. 12.8-12.9 is a fully charged 12 VDC battery. Any voltage above that is just surface charge and will discharge very quickly.
And 12.4 (if measured with the batteries at rest) is a 50% discharged battery-- as low as you want to go before recharging. If the 12.4 reading was with a load on the battery, the battery is in better shape (less deeply discharged).
You don't say what size (in amp hrs) battery bank you have. But with that figure (actually 1/2 that figure, as you don't want to discharge below 50%) and the amp draw of the various appliances and their run time/day, you can calculate pretty closely what you will need to replenish.
