If you're towing then it's almost always best to book directly with the ferry company, or through the Caravan Club, or Caravanning and Camping Club. The 'cheap deals' ferry operators don't seem to cater for caravanners.
Norfolk Lines were doing some good deals, recently, and I did read about someone who had booked with LD Lines from Ramsgate to Ostend for a very reasonable price.
The best time to book a ferry is either late September/October (for the following year) or February - often offers are announced for caravanners, to coincide with the various caravan shows. We booked in October last year, for car and caravan at