Hi Brian and Denise
If the fridge is OK on both gas and 12 volts (whilst travelling) and only trips the electrics, either the circuit breaker or RCD, when switched to mains 230 volts, then it can only be one thing that is causing the problem.
There are two heating elements inserted within the heating coil at different places that come into play when not on gas. A 12 volt heating element which is powered only when the engine is running and a 230 volt heating element that is powered when plugged into the mains. I suspect that the 230 volt heating element is at fault. If it trips a single miniature circuit breaker (MCB) then it has a short circuit and is causing an overload. If it trips the main RCD, thereby switching off all 230 volt power, then it has developed a fault where there is a leakage to earth. This leakage to earth needs only to be minute to cause this particular fault. It is similar to a human being in contact with a live part and the path to earth being through the body. In this instance the RCD is performing as designed, to preserve life by limiting the current to a maximum of 30 milliamps for a prescribed period of time (max 30 milliseconds) and switching off the incoming power.
Either of the above scenarios would mean that the 230 volt heating element requires replacement. It is not a difficult job to replace either element but does mean complete removal of the fridge / freezer which also entails disconnecting the gas supply to the fridge / freezer.
Obviously, any work on gas should be entrusted to qualified person.
Dave (signed in as Christine)