The build

I assume your fridge/freezer will be on the same ring mains as your battery, if full, should be able to keep modern fridge going for at least a week probably 2. We have a 1 kilowatt caravan kettle we can use to draw less power from a battery.

You assume incorrectly.  The fridges and freezers will just have to take care of them selves.  Generally if not opened during power cuts they don't defrost or warm up too much.  As long as power cuts don't last days.  Luckily those sorts of power cuts are few and far between.  If we do have a long one we still have generators