Hi safariuser. These circuit breakers are bimetal strips that open up when they get good and hot. They will re-close as they cool. I don't think they are as fast responding as a fuse or a 120v circuit breaker, but I really don't know. Here is a little description of the devices...
Automotive Circuit Breakers | WiringProducts
I test them like a do a fuse. If I have continuity with my ohm meter, then I assume the circuit breaker is good. You might verify that all the circuit breakers in your box are closed, not open, and have very little resistance across the posts.
A humming converter is a good sign. Mine hums away too. My converter had one heavy 12v wire to power all the trailer's 12v circuits, and one wire to charge the battery. So if you are plugged into shore power and your converter is humming away, I would expect to see something like 13.8 volts across the battery terminals. With the converter off, I would expect to see something like 12.8 volts across the battery terminals. This would be a sign the converter has a 12v output to the battery.
By the way, I assume all your 12v devices run when on battery power only. Airstream was one of the first to make all devices (lights, fans, water pump, etc) 12v. They called this "univolt". So your trailer worked the same plugged in or not. Your 12v system receives power from the battery OR converter. Everything down stream does not really know how its being powered.
Now you would need to test the main 12v supply wire out of the converter. Mine is red in color. Follow that wire along and see where it connects to your Safari's 12v system. Test the end of that wire to ground. I would expect to see about 13.8 volts on my meter. If no volts, then maybe the wire is disconnected somewhere.
It is possible to replace your converter and have the same problem! So I think its important you find the cause of no converter output.
By the way, welcome to AirForums. I have found all sorts of people who have helped me with my questions. And I have found there are no silly questions. Responses are helpful and informative.
A "rivet master" is a AirForums user that has made something like a 100 posts. I see you have earned one rivet already. Soon you will be a rivet master too, and that plus a dollar will get you a cup of coffee!