Usually direct online with P&O Ferries or Brittany Ferries, Could try going out with P&O into Bilbao and back from Le Harve with LD Lines, But i thought it would be cheaper staying with the same operator
I think you may have left it too late to get the bargains. Sometimes the Caravan Club/Caravanning and Camping Club and tour operators such as Alan Rogers, Camping Cheques, or Eurocamp and similar have early season offers, occasionally even on the longer crossings. I'm not sure that it will any difference to the price wherever you book it, now, and some of the 'Cheap Ferry Deals' type websites don't cater for caravanners anyway.