In my experience Rob, and it may or may not be applicable on your side of the rock, built in draw fridges as in Volvo ans Scania are not as efficient as Engel or Waeco chest fridges.
Is it a proper compressor fridge or one of those other electric plate type gizmos?
A compressor type fridge will maintain the set temp regardless of ambient conditions.
The other, less efficient ones will lower the internal temperature X⁰ below ambient, so on warmer days the contents are warmer.