Not a bad deal.
There probably isn't a manual specific to your trailer, most manuals anymore are pretty generic and you get a stack of individual manuals for the various appliances. Many of those are still available from the manufacturers websites.
You may have a bad converter or you may have one that requires that the battery be hooked up to work properly, or you may have a blown fuse somewhere.
I would start by taking a voltage reading at the battery unplugged and then plugged in. The number should be higher plugged in. If not time to check fuses and the circuits.