Check with your voltmeter in a number of places.
First with nothing connected, at the battery.
Then connected to the truck, you should see truck battery voltage.
Then connected to shore power, you should see converter output voltage.
Then most important, check at the converter output terminals both positive and negative.
If you converter is putting out, but the battery is low, likely you have a loose wire between the battery and converter. That is what I found with the same situation, I wiggled the wires at the converter terminal strip, and suddenly the interior lights got much brighter and I had found my problem. It was the negative wire at the converter that was loose, so I tightened it up and all was good again.
