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.
Mike
2012 Winnebago Impulse Silver 26QP
2005 16.6 Double Eagle
2018 Jeep Wrangler JK
previously Snowbird Campers,
Triple E Motorhome and Fifth Wheel