Hi Jon. Have only just noticed this thread. We took our Cali to Iceland on the ferry in May 2018. It was the only thing we booked for our year long trip around Europe. My recollection is that we booked about 5 months in advance. Arriving in early May, there was plenty of snow on the ground (in fact we arrived at Seydisfjordur in a snow storm). However, when we left a month later, spring had arrived and much of the snow had melted. There were plenty of other Calis on the ferry, but once in Iceland, we only met a few on our travels. The one problem we encountered with Iceland in a Cali was the wind. It is a very windy place!! Consequently, there were times when it was too windy to put up the pop up roof. This was a problem for us, since we had so much stuff packed in the van, to cover all climates for the year, that sleeping downstairs was a pain. We did fit an Internal Topper, and also used an External Topper in addition, to help insulate from the cold. I would say that both of these are essential in Iceland at that time of year. We found the campsites fairly basic, some with just toilets and water. Often they were sited close to hot springs. When it got too cold or windy (usually the latter), we resorted to other accommodation. Not cheap, but it gave us the odd night in the warm, and a chance to do some washing.
We had 4WD on the van, but only took one spare tyre, and thankfully never had a puncture. You will have to be prepared to drive on gravel roads, even if not going into the interior, unless you plan to stay on the Ring Road the whole time. We found the north and east of Iceland much quieter than the south, but the whole island is spectacular.
Our biggest regret is that we didn’t opt to stay longer in the Faroes. They are really beautiful islands and well worth exploring. Seeing as the ferry stops there both ways, it is an ideal opportunity to visit this extraordinary place.
I hope that you enjoy your trip.
Heather