The onboard converter is not charging the battery. With a new trailer, the likely suspects are an open fuse or circuit breaker near the tongue, a disconnect switch in the closed position, or blown fuses for the converter itself.
You will need to do some mucking about to find all of the parts of your 12V system. You nay have a junction box near the tongue where various circuits are connected to the battery. That box may/may not have a hidden fuse or failed circuit breaker or a loose connection. The best way to follow through with this is to use a hand held meter.They are cheap and available everywhere. It's a tool that should be in every RV tool box.