Fridge

Yes switch to 12v when driving in most cases your leisure battery will keep your fridge cool

Agree but doesn’t the alternator supply the power via the leisure battery?

No its as baguette said, 3 way fridges work off 220v, Gas ( must be turned on at the bottle ) and 12v supplied from alternator when the engine is running via the engine battery. you have to select which you want.
On newer 3 way with Automatic Energy Selection (AES) you can still manually select which you want, but when set to auto, it will select whatever power source is available, 220v ( EHU ) being the overriding supply, if the gas is turned on at the bottle, then 12v from the engine will override both
This is another reason why some would suggest, you really shouldn't run your engine while connected to a live EHU because you have the engine 12v going against the 220v.
Also be aware that the, Automatic Energy Selection (AES) switching system is actually powered by the Leisure battery, so if you loose a connection from the Leisure battery the Fridge is likely to stay on the last supply it detected. ( So What You Might Ask.)

When I went out for the first trip in our Van, it was late afternoon, so we parked up and went for a walk, when we came back to the van we had no Habitation power, there was a light in the fridge, and we had gas but the spark didn't work, so we lit it with a match, had dinner and went to bed. Next morning we had no gas and when I tried to set the sat-nave we had no ignition, engine battery dead. What we didn't know was, during the drive down the wiring had become disconnected from the output of the Electro-block , which meant their was no power to the AES switching system, because of this the fridge did not disconnect from the engine battery and killed it over night, the gas crash valve closed, ( it is also powered from the engine battery ) the fact that there was a light in the fridge, and a green led was was on, had I notice that it was in the wrong position, still showing connected to the engine. I could have switched it to gas manually.
I know RTFM, but it doesn't show any of this information.

@voyager
"AES fridges always run off the leisure battery for a time period after the engine is switched off before switching to gas. This is to prevent the fridge sparking / firing onto gas when the motorhome may just have stopped at a station for refuelling."

I'm not sure about that, I'll have to check, I know their is a waiting period after the engine is stopped before it fire's up on Gas, for safety reasons , but I understood the fridge does not run at all, for this period.

When the engine is switched off, the display on our Dometic AES fridge continues to show the fridge is still operating on 12V. That was the reason for my statement above.

Today, I ran a test to see what was happening. Started engine (no EHU), then using a clamp on ammeter measured one amp feeding the leisure batteries from the alternator. Switched on fridge on AES. It switched to 12V as it should. Again checked the ammeter, and still one amp feeding the batteries. This confirms what is stated above that the alternator feeds the fridge with 12V direct.

I then switched off the engine and while the fridge display still said the fridge was on 12V, no current was being drawn from the leisure batteries. So, Baguette & Sprinter are correct.

Every day is a learning day.

Thank you for correcting me. Every day is a school day.

" Every day is a school day."
And experience shared is the bases for knowledge, you learn by finding out why you have problems, and knowing what the symptoms are and then finding how to fix it, and if you then share that experience, that's the basis of a community.

I had a look at my van yesterday just to be sure, I disconnected the EHU, switched on the fridge to Auto with the gas turned on, and then ran the engine, I left it warming up while I did a few bits. The fridge panel showed the 12v led lit, when I stopped the engine, this 12v light stayed on until the ignition shut down ( radio switched off, Etc,) this usually takes about 30 sec, after 15 min approx the Gas fired up.