Don't know from a folk view as such but I have been told that the Camping & Caravanning Club have some sort of a deal going with some of the ferry companies for a discount along the lines of if towing a caravan you don't pay extra you only pay for the car. I have no idea if there's a discounted rate for campervans, but here's the Camping & Caravanning Club website .
There may be more info there