The trick is, make sure the inverter is set to "charge only". That forces the fridge into 12vDC mode, which is more efficient in its use of power. By doing that, I've started the fridge to let it cool down as much as 36 hours in advance of the trip, without depleting the house batteries below 65%. Side note— I don't usually cool down the fridge that long; my departure was delayed on that trip. Usually, I'll start the fridge the evening before, and pack the fridge the morning I leave, so it's only on 12~15 hours in advance.
Anyway, the way the Nova Kool fridge is set up, as long as there is a source of 120vAC power, the fridge automatically switches to 120vAC even though it's less efficient in that mode. Any
source of 120vAC power, including the inverter. So the only
way to keep the fridge in 12v
mode is shut the inverter off.
I've debated unplugging the 120vAC cord altogether and using the fridge as 12vDC only, but I haven't been motivated enough to tear open the galley cabinets to reach the electric outlet. Even though someone kindly told me (here on the Forums) how to remove the microwave so I could reach the plug for the fridge.