Radar the van is only 6 months old.

Just reporting back....

So I hooked the car up with the van (Just the wiring not the coupling) took the fridge vent off to get to the wiring at the back of the fridge (see pic attached) Armed with my multi meter started taking measurements with the car running and not running.
Now as I understand pin 1 & 6 shows volts from car battery .... pin 5 &10 is ignition switch from car and pin 3 & 8 is the caravan battery.
As soon as I hooked up the car the AES thetford fridge in the van powered up. Voltages at the back of fridge....
Car battery 1 & 6 ignition on and fridge on
Under load 12.5 to 12.7v ... no load 13.3v.

Ignition off .... Fridge turns off
Car battery 1 & 6 13.1 to 13.2v
5 & 10 ignition switch 0.0v with car off. With car on 13.9 to 14.0v
With car running aux battery read 13.2 to 13.3v while main battery 14.1v (bit of voltage drop there)
Previously the mobile 12v guy who first worked on my set up stated that the van had to be physically hooked up to create an earth .... i proved today that this is false and i will be letting him know this. .... DOES NOT need to be hooked up for fridge to operate.

Radar wrote:

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My reason for that question is, our 2012/2013 caravan had wiring for the frig to the Anderson plug but at about 4 years old it started to play up, I discovered the earth copper wire was all blackened causing us bad earthing. The wire was inported from China and had aluminium and copper mixture, that in DC use don't work, the wire oxidises.

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 It probably was copper plated aluminium. A corrosion time bomb! & the wire is brittle.

Due to more resistance in aluminium you basically need to go to a gauge larger, or even a whisker more.

 

Putting corrosion issues aside. If one planned using 6awg you should be using 4awg.

 

One really needs to get wire from a reputable supplier to make sure it is actually copper.