If it’s the type of fridge where you have to press the button on the fridge front to light it it may be the battery inside the switch.
My Dometic fridge igniter switch has from memory a Aaa battery. Which is inside the switch itself, you have to twist the button to get it out although sometimes it can be stiff.
If you have a manual the instructions are easy to follow
Looking at the diagram for the control box, I assume that this is one of the automatic fridges which switches according to the available power source. Can you manually select it to run on gas if you're connected to EHU?
I would check the 12V supply from the leisure battery. I assume that it must have a 12V supply to the control box at all times.
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