Check the AC electric heater element that's mounted in the exhaust flue for continuity. If you get a ohns reading, turn the fridge to the AC mode, and see if the heater warms up. If it does'nt, check the fuse on the control board, and make sure the wire connections are clean and tight. If all this checks out, replace the control board.
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My apology, I misunderstood the problem. Since the fridge does'nt cool in any mode, I suspect that the refrigerant has leaked. The sealed system uses ammonia at about 300 psi, so it will leak through the smallest break. You need to locate the leak and repair it. Then the system will need to be recharged. This is best done by a RV refrigeration technician.