If you can, fix the anchor to the side walls, not to the center of the back plane. Refrigerator walls are often sandwich constructions of plastic or metal sheets with polyurethane or polystyrene foam in between. These sandwiches as a rule do not like out-of-plane loads. It's better to put the load in plane with the side walls.

I hope that the spot you've picked has enough drainage... And you might want to put a couple of plywood plates on the sides in front of the fridge to prevent the ground around the entrance from collapsing.

I used to help move mobile homes... Just seeing those dirt screw anchors makes my back ache!

I have been to Kings Furniture & Appliances here in town, they have a shop next door for selling odd & ends. I know I can buy for about $15 refrigerator and freezers that don't work anymore. I checked into it when I first read about your fridge root cellar attempt. anyway, have you thought about installing 2,3 or 4 of these in the same spot at once. Since your Gardening Harvests have been growing each year, maybe doing this all at once would save you time in the future. Love your garden stories, your photos of the animals and plants. I hope you guys are successful and never stop blogging about it.