Hi Deborah.
There not very many sites on Arran. There one at whiting bay, and a cl site at shiskine. And another site up at Lochranza. None are caravan or camping and caravan club sites. Apart from the small cl site at shiskine think its a camping and caravan one. Cost for the van on ferry is the same as the car, or depends on how big a van you have. In other words you paying to take two cars over. But if you not been on Arran before its a lovely island but please be aware the roads are not very smooth shall i say so care must be taken with a caravan .
Hope you go as i think you will enjoy all the same.
Dougie.....