Campsite/parking space near Harwich

Hi all,

I'm planing our journey back to Germany. Is somebody knowing a campsite next to Harwich? We would like to go back to Germany by ferry Harwich - Hook of Holland, the ferry is starting at app. 9 am, so we need a location for a rest of one night. Is there a campsite next to Harwich, or is it allowed to stay for one night at the port, on a parking space at the motorway? We did not need much luxury for one night.

Thanks & greetings from Germany, Patric