Nothing Dover to Calais, sadly. These days P and O hardly seem to make 'special offers'.
With DFDS Dover to Dunkerque you can get a reasonable deal through the Caravan Club, where on certain dates and certain sailings the caravan travels free - but you have to buy a package of a ferry and 7 Camping Cheques to get the special ITX fare. Camping Cheque themselves are also doing a cheaper deal with DFDS - details on the website
www.campingcheque.co.uk. Unfortunately this is nothing like as good as the return crossing prices DFDS themselves were offering with the special code at the NEC which was £58.00 for car and caravan return crossing in low season, or £78.00 high season!
DFDS MAY announce another offer at the Excel Caravan and Motorhome show in February, but this is not guaranteed, particularly with the recent closure of the SeaFrance operation from Dover to Calais. There is some suggestion that DFDS may take over their sailings, but I don't think any definite announcements have been made yet.