Blockage to the kitchen sink

Hi, the kitchen sink in our caravan does not drain the water without the help of a plunger. It has had drain cleaners and has been rodded from the sink down to check for blockages. The other waste water (shower and basin) drain without a problem. The van is an Eldiss Avante 524 and I am certain that it has an airlock in the line but am unable to access it as the fridge is underneath the sink and I am unsure how to remove it without causing more problems for myself. Has anyone else had a similar problem an found a solution?