Why not just park somewhere else, away from the camper and any other potential sources of polarised light and see what happens.

Thanks again for all replies. No, not on Mars (well, bars maybe..) Not near Street light. 

Jim to be perfectly honest I can't be positive if it was the motorhome side or not. That rear side window isn't concave, but as it is the bathroom, I fitted one-way window foil.. Now, more to the point though, all this has happened since moving the motorhome to sit next to the Yeti. It was previously parked on another part of the drive. Is it conceivable that the American gel coat on the 'van can reflect the Sun's rays to that magnitude one wonders? John.

It must be a Concave surface in order to focus (and therefore concentrate the heat) an image of the sun. A dent on a very shiny surface could have this effect for example. A flat surface would reflect the sunlight but that would only have a similar temperature to the sunlight anyway.

 

Is the foil on that window concave (as viewed from the outside?)

I'm sure the window is the cause :

Quote

Low-e and energy efficient windows are usually the cause of the sun's reflection burning grass. The glass in these double paned windows sometimes warp and becomes concave due to the barometric pressure between the interior of the window panes and the air pressure outside. This concavity focuses the sunlight reflected from the window pane in the same way that a magnifying glass does. When the light hits the vinyl siding of an adjacent wall or the artificial turf, it can cause the materials to burn or melt.

 

 

https://windowfilmforturf.com/blogs/news/understanding-the-dangers-of-sun-reflection-on-artificial-grass

 

Jim