I have used a similar setup with success, though my winters are slightly warmer than yours. Most winter nights are in the 40s F with only occasional high 30s here. I'm not sure if the fleece will provide much more protection than the plastic is already giving them, but you could throw it over the top as just an extra layer. I would also add a heat source to raise the temp at least a few degrees. Some people will use strings of Christmas lights for example, the old kind not the LED ones, that put off a little heat. I use Snuggle Safe pet bed discs that release their heat over several hours (available on Amazon) and just set them on the bottom shelf. If you have space to move the greenhouses up against your house, that will also help them stay slightly warmer.

Good luck!

I leave my succulents outside all winter in those temperatures and they do fine. I only move them inside if there's a chance of frost.
Yours are pretty well sheltered against that wall and I would only cover them if it might frost

Thanks Nik and Sara for your advice. Some kind of heating source is a good idea. Perhaps heat mats for seedlings? Gonna give it a try!

Keep 'um dry. Some people here when it dips slightly below 0 C use a string of incandescent holiday lights inside a plastic tent. The incandescents add just enough heat to keep the plants warm. Be very careful the bulbs don't touch anything including any foliage and the plastic tent.