Originally Posted by tomk
sounds like a key-off ("parasitic") drain..
I agree. There is some type of electrical gremlin or short that is draining the battery when the vehicle is sitting. With so much electronical and electrical equipment on these vehicles, God only knows where it is. It must be a fairly large drain or short to kill it completely dead overnight. Also, remember unless the battery is a deep cycle marine type battery, a complete discharge to the point it's totally dead can and will ruin a battery. Even a brand new one. Their testing may be correct that is showing the battery is bad. But they are stopping there instead of finding out WHY.
If they just put another battery in it, tell them to keep it overnight. They'll find out real quick if they solved the problem or not.