Answering questions about how cold it is can be tricky, as people’s tolerance for cold can be very different.
You can find highly detailed and more scientific information about the Icelandic climate in this article by Markús Einarsson.
The short answer is that most people never have major issues with the cold in our camper vans, especially during the summer months. In the winter, we suggest bringing/renting an extra sleeping bag and obviously a lot of warm clothes. That, in addition to the included bedding and heating system, keeps most people warm enough. If you are highly sensitive to cold temperatures, we suggest that you make special preparations to stay comfortable.