12V Inline System Fuse

Protection should be near any power source. Generally that means the line is protected at the battery and at the converter, they are both power sources.

I'm not sure what you are calling the "12V electrical box".

OK...thx everyone.

Yes...I should have said Circuit Breaker.

There is one 12" from the battery positive and then another about 18" from that downstream near the 12V electrical box.

No electric stabilizers.

Best practices is to have circuit protection within 18' of the positive battery terminal. Someone my have upgraded that cable and followed that schedule. You only need one circuit breaker to protect the main feed from the positive battery terminal.

If they are in series between the batteries and the "electrical box", no need to have two of them.

The main reason for the FUSE at the battery terminals is to protect the wiring from a direct short to frame ground down the leg. The battery(s) can produce a very large amount of DC current for a short period of time... Could cause a fire from burning cables or the battery itself may even blow up which is a rare thing...

I had to change my 12V IN-LINE fuse vales when I upgraded to more batteries as my increased loads was more than the fuse value...

Roy Ken

I would guess that the 30 amp fuse is protect the wiring in case of a short. The 6 amp circuit breaker would protect the motor from overload. The motor would draw more than the 6 amps up until the breaker heated up enough to open the connection.

Typically the first line of protection is installed within a very short distance from the battery to protect in the case of a short in the line and overheating the wire. Don't know why they would have installed another in series with the first.

Do you have electric stabilizer jacks? If so, Lippert specifies a 30 amp fuse and 6 amp breaker in series. The manufacturer of your TT (if you have the elec. jacks) may not have followed Lippert's specs. They didn't in ours.

Actually what your describing is a 30A auto reset circuit breaker, not a fuse. Fuses are a one time item.
Do you need two? No, but I suspect one is to protect the battery, the second one to protect something else. Only way to know for sure is to disconnect one and see what does not work.