Hi
@geedub , sounds like a good set up!! But a couple of questions for you. What happens when you stop the engine?? I assume you have some sort of auto switching to isolate the fridge current draw from the the house battery and the second battery in the car. Obviously, you have a two battery arrangement in the car to isolate the primary battery as well when engine stopped. In your set up, you are getting full alternator voltage of 14.4 volts to house battery and this voltage is also available to the fridge. No wonder it operates so well!!. In my Penguin, there are three wires via Anderson plug from my tug to the van. The two heavy ones go directly to the fridge (via 20A fuze) and the fridge has NO electrical connection to the house battery. The third wire is a lot thinner and goes to the house battery via the Setec to keep it charged whilst engine running. Unfortunately, it has to pass through a power diode which prevents the current going back to tug. This causes a loss of approx .7 volts, so even if tug alternator output voltage was 14.4 volts or similar, by the time it gets to the van battery, it is down to 13.7 volts, which is just "float" voltage for an AGM battery. I also only have the tug's starter battery, plus a 120AH AGM in the van, but I can see the value in what you have done. Your system circumvents these hassles and keeps voltage high at the 12 volt heating element.
If some or most of your batteries in tug and/or van are a little low, and the fridge was on too, your alternator would really be working to get some juice into these batteries.
I don't have any freezing problems probably because the fridge never gets cold enough on 12 V. Although it can and does on gas or 240 V. I no longer have freezing problems then because I have installed 2 computer fans inside my Thetford fridge which also keeps the ice off the cooling fins, besides keeping fridge temp more even.
I only have thermostatic control of fridge temperature when on 240 V power unfortunately.. You would need a really heavy duty thermostat to control fridge temp whilst on 12 V as current would normally be around 15 or 16 A at 12 volts, but with the higher voltage of 14.4 approx, it would be down around 12 A.