Dometic fridge,

Your truck may be equipped with a protection routine inside it's master computer that denies any electrical drain from your batteries when engine is off.

Your fridge must be considered a temporary short term storage of food. No more than a couple of days max with engine going. If the engine is off, take the fridge out and plug in it's 120 volt cord to a building or parking lot lamp post etc.

I don't know your particular brand and type, we only had a generic dual 120/12 v fridge and worked with it's limitations by poaching off building or lamp posts when truck is in shop etc.

 

Your truck has a LVD (Low Voltage Disconnect)

When your batteries drain down to a preset value, (usually 12 volts) on the LVD, the LVD will shut down certain circuits in the truck so you will have enough voltage to start the truck.

The reason it may never have done it before was your batteries were stronger back then, also since it's summertime, the batteries take a beating with heat and are considerably weaker.

 

You better load test all 4 batteries prior to winter.

If you think those things take a beating now, wait until -30 and ice etc.

It's been my never ending experience with dying batteries. They always give a early warning, a sort of cry for help in the smallest of ways such as low voltage before it really goes down.
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