Shipping length of the caravan includes the drawbar. shipping length length of the car includes the towbar. However, if you add the two together you may find they come to less than 12m, so if booking a ferry, "adjust" one of the other to get down to 12m or somewhere very near. since most ferrys go to cars up to 5m and vans up to 7m you are buying a 12m space, so don't be too modest about it. particularly in winter the odd cm. or a few is very unlikely to get checked.
Probably get banned for saying this but good luck anyway.