You haven't confirmed whether or not you have load tested the battery. If you have a dead cell it may show 12volt or more quickly as you connect charger or immediately after running vehicle. However it will drop as soon as any significant load is applied. A dead cell can make it near impossible to jump start with either another vehicle or charger. I have bought new battery's off the shelf and had battery failures. Your local parts store would likely test your battery for free.
