Our fridge does not cool down on 12v, but does keep it at the same temperature for the duration of the trip whether 4 or 5 hours. Definitely will not freeze anything, but will keep frozen foods solid.
I thought it was the amp and not the 12v that counted towards it cooling and I am not sure what sort of amp a solar panel will generate.
I assume that the tech when they fitted the electrics for the towbar fitted the correct wiring for the fridge. We have not used the 12v for quite some time, but taking into account this thread, we will be doing so tomorrow on our trip up the M5.
Apologies for not making it clear. We cool down the fridge to maximum before a long journey i.e. travelling to port, trip to continent and then the drive to the site.
When fridges cool down the temperature initially falls quickly but the rate of change reduces as the temperature difference to the ambient increases, In fact the rate of change is an exponential pattern. In addition most fridges do have a limit to the temperature difference they can create due to the formulation of the refrigerant gas they use, so most fridge manufacturers will quote a maximum temperature difference relative to ambient.
If you have pre cooled the fridge before connecting the 12V supply, then even though the 12V systems may have some capacity to further cool the fridge, it may only be a few degrees cooler at best depending on the ambient conditions and it would take considerably longer to cool the last few degrees than it does to drop the first few degrees.
But as soon as the caravan is in motion the air flow around the fridge caused by towing aerodynamics will disturb the effectiveness of the cooling system and it may prevent any further temperature reduction. It's all very installation dependant, so some installation's may be better and some worse.
The combination of the exponential cooling curve, and motion air flow disturbance and the limited time of operation on 12V power (basically only whilst towing) may prevent the 12V operation from producing any significant additional cooling. But if the fridge were stationary and operated on 12V with an adequate 12V supply, it would demonstrate very similar performance to either mains or gas operation.