Check on your converter right on the positive and negative output. If the voltage stays up there, but drops at the battery terminals, you have a loose wire between the two. In my case, it was the main negative wire on the negative terminal strip on the converter.
The voltage gets drawn down by a loose connection, it makes it seem just like a load.
Try connecting a jumper lead to your battery negative, then to the negative strip on the converter. If that fixes it, is is the negative wire. If not, try connecting a jumper lead to your battery positive, then to the positive strip on the converter. If that fixes it, it is the positive wire.