This usually happens when I'm at some event in summer and there's no space in hotels so I camp. In the day I leave the sleeping bag(s) in the trunk of the car in a big bundle and bury anything sensitive to heat right in the heart of the bundle. The sleeping bag stops me losing heat at night but it also works the other way and stops heat getting in during the day.

The most vulnerable element to heat is probably the Lithium-Ion battery (don't forget the spare as well). Lithium-Ion batteries age very quickly if they get hot.