To get mine to an operating temperature, I use propane, and I use either hard freezer gel packs, or frozen 1 liter bottles of water. One of those in the freezer compartment, and two in the refrigerator compartment do the job at keeping cold stuff cold as soon as it gets put in.
I also have a small fridge fan that uses two C batteries. This keeps air circulating around the inside.
I usually get the freezer to operating temperature within 8 hours. From there, I can take the freezer pack out of there and put it in the fridge, and by the time that warms up, the refrigerator is good to go.