my dometic fridge

Absorption fridges ALWAYS take a long time to cool down but it it won't STAY cool then I'm betting there is a mechanical reason for it. Flame not lighting (when operating in LP mode) or heater not working correctly when in electric mode. This could be caused by bad thermostat or could be something as simple as a bad door seal. Also if you run with the climate switch turned on AND a bad door seal you would likely have a hard time keeping it cold. Knowing when and under what circumstances it warms back up again would be a help in diagnosing the problem. If it fails when moving, it's probably the wind blowing out the flame (with DSI ignition it SHOULD re-light but if it's a pilot light model that can be a big problem. Just not enough information to give much meaningful assistance of a specific nature.