Fridge fault on 12volt operation

PennysOwner

My fridge does not operation when on 12volts with engine running. A noise like a  switch opening/closing comes from the fuse box and fault intermitenly appears on fridge display. Notice also the 12volt lights come on and off at same time. Anyone any ideas
as to what might be wrong.

Caravan is a Coachman Amara 520/4 (2009)