A rather late update to our progress on the electrical situation with the motorhome. After replacing the fridge, the freezer and the solar controller things were looking up. The batteries were charging fully and the solar system was maintaining the power while just the freezer was running. As soon as I introduced any other load the system would fall over. I had bought a battery tester online and the one that I got measured cold cranking amps – not the best way to measure deep cycle batteries. In any case it said the batteries were fine but I was starting to have my doubts so I bought another battery tester that applied a known load (100 amps) to the battery and measured the response. It cost me a grand total of $24 which would have saved me a fortune if I had bought it first. The test showed that there was a faulty cell in one battery and that the other was getting towards the end of its life (which only figures as it had been carrying most of the load for who knows how long). I ended up replacing both batteries and slightly reduced the amount of storage (270 Ah compared to 300 Ah) to improve charging time. This seems to have done the trick with the fridge and freezer running on hot days without fully depleting the batteries. My tests showed that we still had 70% capacity following a hot, overcast day.
Only on the road trialling will tell us the true story so I guess we will wait until then for real confirmation.