Thetford N80 3 way fridge not getting cold?

The fridge had been standing for 2/3 years after we acquired this accident repair damaged caravan.
We were advised to turn fridge upside down for long periods, shake it and switch on at least 12 hours before use. anyway to no avail! the electric and gas works on it, but the condenser inside fridge doesnt work so fridge does not get cold.
has anyone any suggestions before we take it to a local thetford engineer.
Many thanks Sara