From the point of view of the selection of the power source, there are two types of trivalent refrigerators: those with manual selection, where you set the type of power supply, and those with automatic selection that set it automatically (it is better, however, that they also have the manual position) .
How to select the power source?
Easy:
when you are connected to 220v with the cable, put the fridge selector on 220
when you are parked but you are not connected to the 220v, put the selector on GAS
when you're in motion, so with the engine running, put the selector on 12v. In this position, the fridge is powered only when the engine is running, when you stop the fridge switches off (if the stop is short, ok, otherwise switch it to GAS and remember to switch it back to 12v as soon as you leave).
Obviously, if the fridge is automatic, it will choose the power supply by itself.