In reply to Rog Wilko:
I've just got back from a trip to Iceland, though we were in tents rather than a van. But there was no difference in price between a tent and a van, you just paid a set price for each person. The most expensive campsite we stayed on cost just under £9 per person, so probably not worth bothering with vouchers (didn't come across any anyway).
Have you rented a 4x4 van that you can take on the highland roads in the interior? Well worth it!
It was clear that some camper vans were staying on roadsides rather than in campsites, but we weren't on the main roads much, so no idea what the authorities might do there.
The road.is website is a great source of info for roads, weather, temperatures etc. as well as indicating whether the roads are 4x4 only or ok for any vehicle.