Originally Posted by Vermilye
The 30 amp breaker goes between the batteries and the converter. Without pulling my bed apart I'm going by memory on which side of the 30 amp breaker the tug feed connects, but I believe it is connected to the battery side.
That would mean it is protecting the wiring between battery and converter from being overloaded by the battery supplying more than 30 amps to the total of all of the trailer's load circuits, when the shore power and converter are not on. That should be a rare occurrence, so the breaker should last a long time.
It would not be primarily for protecting that same wiring from the converter supplying too much current to the battery, since the converter itself and its output fusing should limit that, presumably to the same value... although in this case the converter does appear to be capable of supply more than 30 amps.
Since the converter output capacity is 55 amps, there seems to be an assumption that the full converter output will never charge the battery (that 55 amps will only be reached by a combination of charging the battery while running significant other loads).