S+ connector on Dometic fridge.

[Reallyretired]

I assume that the wire from the S+ on the solar controller to the S+ on the fridge is just a signal to use the surplus solar and so a thin wire will suffice. The actual power to the heating element coming through the existing wiring from the leisure battery. (Hymer so fridge can run off 12v when engine not running)
I have previously just switched manually to 12v when there is excess solar, but doing it properly will mean no risk of leaving it on by mistake.