converter or battery

Getting the fifth wheel ready. DC lights bright and then quit working. Open electrical panel and tapped on it and lights came back. A minute later lights went back off. Tried tightening a few screws and wire nut lights came back on. Went back off a minute later. They do not dim, are either bright or off.
I know the battery is bad but shouldn't the converter run the DC lights?
I did hear the converter fan run a couple of times.
Lights work great with external battery charged hooked up.
I cannot pull the converter out far enough to check for loose wires in back.
Any thoughts will appreciated.
Will be buying a new battery tomorrow but not sure how to check the converter and hate to discharge a new battery.