That power center is pretty common -- and reliable. Door just swings down.
With no power at the battery terminals there's a fuse or other break between those points. Most common in my experience is the battery protection fuses in the power panel which will blow if the battery is connected backwards, a common error due to misunderstanding 12v "house" wiring codes vs 12v "vehicle" codes.
While the battery is out is a good time to verify that wiring. A sticker on the trailer tongue usually lists what color goes where. Find the wire that's grounded to the trailer frame and note the color. That's the Negative wire (regardless of color).
There is also normally a 12v protection circuit breaker in a metal box under the front trailer. This usually just protects the the vehicle battery charge circuit but will also disconnect the battery.
Here's the power panel you mentioned. (The converter is buried inside it.) Pretty simple and only a few circuits.