Well , I would avoid whereas possible leaving the car in Algeciras , as is a high-crime city (as most harbours-cities , actually ) .
If you can not find anything better, you could park your car/trailer in Marbella/Pto Banus (about 30-50 kms from Algeciras ) . There you have several public underground parkings , with CCTV , security staff , quite safe actually, etc , at a not-that high rate . You could probbably negotiate a weekly rate , etc ... .It quite a posh city ,fairly safe , "tourist friendly" , English and German is widely spoken , and being most cars around from abroad , your car won't take undue attention neither.Id say your car would be there as safe as a car in a NCP parking at -say- central London.
Your second (and more risky ) option is the parking at the ferry-terminal itself of Algeciras . Its got CCTV , and watchmen, also , with all the convenience of being literally "over" the ferry queue.Its more cheap , and -while still safe- , could be a bit frigtening at nights. The London's safety-equivalent would be a open parking lot at Sainsbury . I wouldnt be surprised if you find something missing (stereo , headlamps , etc.. ) in a week.
By all means!!! do positively NOT leave the car/trailer in a public street in Algeciras ... or even worse in Ceuta !!!!. London's equivalent for leaving a car in Brixton with the keys on the ignition.
If you dont find anything better , Id definitely go for the Marbella Option. Any street of Marbella is much safer than any parking in Algeciras .