One of the things you said can be interperted two ways
We checked the house batteries and all four were 6.92
Now, House batteries are always 12 volt (Resting full charge is 12.6) never 6 volt.. HOWEVER a 12 volt battery can be made by wiring two six volt batteries in series.
So do you have 4 12 volt (Six cells each) batteries or do you have 2 12 volt batteries each made from two Six Volt (3 cells each) batteries?
The answer to that makes a BIG difference.
I'm guessing you have 4 very dead 12 volt Marine/Deep cycle and likely will need to replace them because they tend not to recover from that level of discharge.
on the other hand, if my guess is wrong, they are full up.
