Ok.... could be off,... but, if the new bats were in fact up to par,... then...
I have seen some wierdness where... a new car was bought, then, had starting problems then, was dead. WIth that, in 5 months, 2 new batteries were replaced. Charge (alty) was ok. Again, a dead battery issue arose. Car was toed to the dealer. A real tech dug, and found one HOT lead wire had a spot were it was cut, and slightly making contact to the frame. It caused a very slow drain or draw on the cell. Over; time, like, a couple months, it killed it.
But, it did cause some corrosion on the post, (why i asked) due to a low connstant draw. Look,.. im just saying it was a terrific find. Or, the dealership woulda just swapped a new battery in every 2 months. So.... a meter, and dig on the cables bro......Especially if your 8v is true.
Normally a dead bat wont even make it that low. Know what i mean? If in fact youve seen 8v, then, there is a constant draw........well, maybe to me.....
EIther way, and maybe im way off..... keep us informed bro.. good luck