12.4 volts is way low for a fully charged battery.
The simple way to test your battery condition is to go to an auto store and buy one of those "turkey baster" battery testers assuming you have wet cell batteries.
How are you charging the 2 present batteries? Is it a 3 stage charger. Does it bring the batteries to 14.2V or more when charging initially?
Lots more to ask...but get the turkey baster (hygrometer) first...about $6 bucks.