A standard camping cool box type unit is not up to the task. We used to carry a separate table top domestic fridge and a table top domestic freezer. They sat side by side in the Yeti boot and offered ice cold beer and 3 weeks supply of frozen food from home.
We now run a single unit. A newer version of it is marketed by Dometic as the DOMETIC CK40D HYBRID COOLBOX by Leisure Shop Direct.
We considered a 3 way but this caught our attention for it's freezer capability. On mains voltage it will supposedly run at down to - 15c. Last year in NL with 34/36c it was holding - 12c during the day in a shaded location under the sun canopy.
On 12v in the car, after leaving home reading - 15, it rose to - 10/12 over a 16 hour journey including no power for 3 hours on the Channel Crossing.
On 12v it is listed as dropping 20c over ambient, so perfect between sites in your air conditioned car if being used as a fridge.
We are very impressed with the performance and bought ours for about £200 a few years ago. The model being marketed by Leisure Shop Direct is now reduced to £344.It is a lot of money, With a hour or so spent on the Internet I'm sure you could find the model we have without the digital display a little cheaper. I would definitely buy another.

We have the newer generation fridge fitted to new Tourings and our Feeling. At present we are experiencing the same or slightly higher temperatures than you, about 50 miles to your east. Day time temps have averaged 34/36c, topping out at 38c. In our caravan fridge I'm not keeping any meat products or beer. It is standing up well to these high temeratures with the exception of milk in the door that is not as cold as I'd prefer. I do not think the cooler air is circulating as well in this area as the shelves are fairly full.
After 6 hot days I've taken outside vents off and sat our small fan next to the bottom vent to see how effective it will be I lowering the temperature.
I probably will now add a couple of fridge fans as on previous vans as it will only take a morning and cost less than £20.

Colin

Our 3 way Electrolux Camping fridge has been with us for 21 years now and lives outside, full of ice cold gin, tonic, vodka and beer. Oh and a bottle of pop.
When we were true campers we also used to carry a common or garden table top fridge in the tent.
Since the Eriba we have ditched the table top fridge in favour of a table top freezer which once powered up will work well and survive the 4 or 5 hour downtime between campsite stops.
R

They do look like a good solution to the problem.

Colin