You need to hook the panel to your main battery if you stick with one battery. Trust me I work in the industry.
If you chose to mount a panel for instance to your roof rack and connect it to the main battery and then the fridge off the main battery, you need a panel and regulator.
While the engine is running the solar regulator disconnects but reconnects once the car stops and you have sun to power the panel. A 120 watt panel produces 6.6 amps max and is enough to keep your battery charged and run the fridge while stationary. I know this because I had this set up to begin with and I use a 60 litre Waeco fridge.
I now have dual batteries but still have the panel etc.
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