How do I know if my Charger, Converter is bad? Pics inside

The picture isn't there.

Regardless, your logic is sound and I think your diagnosis is probably correct. There's always the chance of some other problem, a connection between the converter and the battery for instance, but I think you've ruled out the usual suspects other than the converter pretty well.

It may, of course, take a reasonably long time (hours) for the voltage to rise to the full output level if the batteries are discharged a fair bit, as they would be at a 12V resting voltage.