How to get my cooler system to work on my N80ELL thetford caravan fridge?

the cooling condenser is not working in our fridge. it works on 12 volt , electric and gas supply, but fridge does not get cold! we've turned it upside down, shook it about, tied it up in our van and drove around so vibration would unblock crystals ..... even took to thetford engineer and he said its beyond repair. well the fridge has never been used. we bought the caravan with accident damage on front, all appliances work accept this fridge... help. a new one around £600
Many thanks Sara