The first time you deep discharge a starting battery, it will never be the same again. If you bring it in for a "test" batteries can take a surface charge, and indicate a carrying voltage that appears to be high enough. In order to test a battery properly you need a real load on it.
So the first problem is something is causing a parasitic draw on your starting battery, which permanently damages the battery from taking a full charge in the future.
This then begets other problems:
battery can't take a charge causes inadqueate cranking speed, makes engine not start, and puts lots of strain on starter motor.
If it does start, the alternator is working overtime to sink current into the battery that won't take the charge. Instead you just lose the alternator output to heat.
Put a known good* battery (* a battery that starts other vehicles, not one that "tests" good) and see if vehicle starts. If it does, next check for parasitic draw. With everything off on the bus, put a multimeter in amperage measurement mode across positive terminal. It should be ZERO amps. If it is not, the battery will run down.
You will need to find where the draw is coming from. In vehicles with auto resetting circuit breakers, you might need to unscrew the circuits one at a time and check for draws. If you have fuses or manual breakers, trip one at a time or pull fuses until the draw goes away or changes.
Chase down each draw and determine if there's a switch or a light or something pulling current. It could simply be something like a bad ignition switch drawing current all the time, for example.
Originally Posted by Forest-Bus
I think we have ruled out the alternator for the most part.