Keeping fridge cold on longer ferry journey

IanG2Chapel

My wife and I taking the caravan over to Denmark next month on DFDS sailing Harwich to Esbjerg. A crossing taking some 17 hrs. plus. Wanting some other peoples ideas on this route please, in particular what tips you may have on keeping fridge cool on the
long crossing. We have overnight cabin all booked so looking forward to pleasant journey. Thought about either: keeping fridge up really cold as we are taking couple of days and stopping at cl on way down to Harwich from the East Midlands (where we live),
OR alternatively, to put a couple of small ice blocks in the fridge prior to boarding the ferry. Heard Denmark is expensive for food shopping so wish to take some food with us to get us going. Thank you for any tips, will be welcome.;