As the fridge generally use an element which draws about 10 amps, you could arrange a battery charger(with suitable capacity) to deliver 12 Volt to a socket where you plug your 12S plug into when it is not connected to the car.
The positive is pin 6 on 12S, negative pin 7. On 13 pin socket pin 10 is positive and pin 11 negative.
My reasoning for getting it running of the EHU is so i can get it cooled down the night before we go away in it.
It would also save using up the gas.
I will be getting the socket wired onto the back of the car soon to run the fridge but as we are not going very far whilst getting the hang of things it wouldnt get much time to cool down.
If it helps the fridge is a
Electrolux Model RM122 (bottled gas and 12 volt operation.
In the manual it says depending on gas control settings the gas consuption will be as follows
1.) 0.15 kg per 24 hours
2.) 0.19 kg per 24 hours
3.) 0.23 kg per 24 hours
for operation on 12volt the boiler on the cooling unit is fitted with a 85watt heating element.
------------- She may be old, she may be rough around the edges but she is our home from home and we love it.
It's much less complicated simply to cool down the fridge by using freezer blocks, freezing food you won't need until the next day and treating it as a coolbox until you get where you're going. I had a two way (the same model) and just put everything I needed out of fridge and freezer in before setting off, added a few freezer blocks or a small bottle of frozen water, then shut the door, then onto gas when I got to my destination.
They don't actually use a lot of gas once they have cooled down.
I have a three way now but still do the same, and never use the 12v bit, as its very ineffective anyway.
In 2 weeks we are going away for are 3rd trip away in van this time for 4 nights at a steam rally so not as much access to shops so will be needing the fridge.
I serviced the fridge and got it running yesterday as it had not been used in 8 years! Nice blue flame now.
------------- She may be old, she may be rough around the edges but she is our home from home and we love it.
Dont bother, a 12V PSU will cost for that size, and the 12V element will only maintain a chill, not create it (if that makes sense)
Stick to gas, or coolblocks. 150-200g of gas isnt much