Bopeep,
We have travelled through St. Malo a number of times but never actually stayed at the docks overnight. We have always stayed at a site called Domaine de la Ville Huchet. It is on the outskirts of St. Malo about 3 miles from the docks (10 mins in the car). We will be using the site again this year.
The site can either be booked direct or last time we booked it through the Caravan Club. They have a number of pitches which are used for overnight stays and last time we stayed there were about six of us who left in time to get the ferry in the morning.
http://www.lavillehuchet.co.uk/
The site is in the CC Continental Caravinning booklet and on their website, numbered B32.
It can be a little tricky getting onto the main road into St. Malo from the site though as you have to enter an extremely busy roundabout and it isn't always easy to pull into traffic with a van on the back. Apart from that it is a lovely site.