Take a negative line from the battery to the negative on the converter. If that makes things work properly, you have lost your negative. If not, remove that line and connect the line from battery positive to converter positive. If that works, you have lost your positive.
In my case when converter output seemed fine, I found a very loose heavy negative wire loose in the converter negative terminal strip. I just wiggled the wires, and suddenly everything in the camper went bright so I knew what was wrong immediately. Try wiggling the wires.