To test the converter output you would need a volt meter.
It doesn't have to be an expensive one. Harbor Freight has one similar.
VoltmeterYou would connect the black lead to ground and red lead to the positive. This can be done directly to the output of the converter or in the 12 volt DC distribution panel input.
If you use the converter output, it would prove that the converter is good or bad.
If using the 12 volt DC terminals in distribution panel and it shows low voltage you would still need to check converter.
You should see 13 plus volts from converter.