It's so easy to check on the internet that it's worth checking
1. With the ferry company direct.
2. With the 'Cheap Ferries' websites (which usually aren't)
3. With the Caravan Club or Caravanning and Camping Club.
Make sure you give the correct details each time.
It's also worth checking a couple of different dates - for example caravans sometimes go free on certain days of the week, and this can make a huge difference.