If you have a Progressive Dynamics converter for example. There are fuses (2) which are there to protect the unit if it is connected with the wrong polarity. Not to protect it from an overload. The large fuses in your fuse panel are there for the protection of the wire between the converter and the fuse panel.
Back in the last century, there were battery chargers that were literally small welding machines. These devices would literally destroy themselves if shorted for an excessive period of time. With the advent of intelligent design of the modern charging equipment. These devices will protect themselves by limiting the output current. The less expensive devices just "crowbar" shut down the output until the overload is removed. Others simply limit the current that is available.
This is the reason I always recommend using a modern battery charger when troubleshooting the 12 volt
DC systems on a trailer. Instead of using a battery. I don't recommend do this in any way. But if you were to connect a wire across the + and - terminals of a battery. The wire would turn red hot and melt instantaneously or worse yet, the battery may explode. You can do a lot of damage to the wiring in the trailer by using a battery that is not wired thru the correct protective device, either a fuse or 12 volt
Probably more than you wanted or needed to know.