The 'trip switch' is an RCD, the two 'breakers' are MCBs. They do different things. An MCB protects against excessive amps, which could cause overheating and possibly a fire. An RCD protects against leakage to earth. If it detects even a very small leakage (typically 30 milliamps) it will cut the power. If you accidentally touch a live wire, a leakage current flows through you to earth, giving you a shock. The RCD will trip to prevent the shock.I’ve not tried with all off actually. I’ve got one trip switch and two breakers. I’ve tried with one off then the other and still trips. Even tried a different cable.
If the breakers are single pole it's a good idea to change them for double pole breakers. All the motorhome manufacturers, even the British ones, fit double pole breakers.If the breakers are single pole switching them of will not stop a neutral earth fault.