@Chockswahay; I went back and re-read your OP and you are clearly (and in my view rightly) concerned about running the fridge off one 85amp battery. If you are going to upgrade to 2 batteries I would strongly recommend you consider fitting a battery monitor at the same time. I fitted a NASA BM-1 Bluetooth monitor which is cheap(ish) very easy to fit and does not need wires running around the van. All the wiring is right by the batteries. Getting the drivers seat out is by far the most difficult part. Fitting the monitor has removed all my battery fears, real or imagined. I felt I was back in control; after all warm white wine really is the pits!