Hi
Many years ago you were allowed to park on the marina carpark. We could see them from our boat!
Last summer when we arrived we couldn't find anywhere - then we followed a
MH onto the Cite de la Mer carpark which is adjacent to the marina and 300yds from ferry terminal, there are no services, it was free and we felt perfectly safe.
Head for Cite de la Mer, well sign posted, follow carpark as far as you can toward end of sea wall.
As I have said this was last summer, but the were loads of vans on there and we stayed another night on way homw to do pet passport bit.