[Charli]

Bought some of the bubble wrap type material from Camping World last winter. It has a reflective foil exterior which works quite well at reflecting heat back out of the window. Is sold in rolls and I cut it to fit each of the windows and placed it against the window and pulled down the day/night shade to keep it in place. Worked quite well and was inexpensive.
We did notice some units, particularly motor homes, had custom made sun shades mounted to the exterior of the windows. Expect these would work better as they are blocking the heat before it enters through the windows. I believe you can still see out the window with these in place which would be a real advantage. Probably a little more pricey. May look into these in the future. Till then it's foil bubble wrap for me.