I have had occasion to remove the microwave in my 2001 Roadtrek 190 P and noticed that there is no insulation behind it. It is just bare fiberglass wall. This section is about 2 square feet. I have also had to remove the vent hood over the stove and there is no insulation there either. All of the insulation around these area is paper backed fiberglass.
The past two nights, my wife and I camped on the Washington coast. This morning we got up to ice on the windows behind the foil-backed bubble wrap type insulation. To say the least, it was a bit chilly. I am thinking about using some of the foil backed styrafoam to insulate behind the microwave and vent hood.
The single pane windows is an issue both in summer and winter. We have cut foil backed bubble wrap for all the windows and it seems to help a little.