Dometic rm2350

Hasn't worked on 12v for a while, was working on 240, but it's not now.

Ok, so there are a couple of things it could be, in ascending order of severity.

1. Faulty thermostat
2. Coolant is bubble locked, by that I mean there is an issue when the coolant no longer circulates correctly through the system.
3. You've lost all your coolant gas which is a mixture of Ammonia and other gasses.

In the old days we would take the fridge out and turn it upside down for a day then put it back in. That seems to reset the flow of coolant.

How a 3 way fridge works

Thermostat usually not involved when on 12v.

Yeah, but some of them do run the 12 volt through the thermostat as well. Mostly later models though!