Have you noticed a gradual dimming? Been plugged in for awhile? I had the same happen, and found a loose connection between the converter and the battery.
The converter was putting out max, but the loose connection wasn't allowing anything to go to the battery. Lights were getting dimmer.. idiot light on the battery sensor had been dropping, should be at max while plugged in.
When I hooked up the truck, all the lights came on bright. Different feed wires from the truck to the battery.
Measure converter output right at the converter terminals. Wiggle the terminals and wires with lights on to see if suddenly you find the connection that is loose. That worked for me.