Hello Andy, I think it is very unlikely that you will find any practical method of keeping a caravan fridge working during a ferry crossing. Gas has to be turned off, there is no 2302V available, and teh fridges consumption of 12V will flatten even a big 110Ah battery over 11 hours. Best suggestion is to stock it up well, chill it down as cold as you can before the journey, and dont open the door. You could pre chill some cold blocks in the home freezer and pop them in which may help. If in doubt dont store any meat, but buy it at your destination.