Not fully charging batteries

[Trey7981]

Ok, my truck has 8 batteries. 4 for the truck and 4 for the epu I replaced the 4 for the truck as that was the bad batteries location. Should I replace the other 4 as well or is that a separate system. And to be honest I did notice an overcharge one day about 15v then it went to 13.2-13.6v the next day. So most likely my alt is failing is what your saying correct?