Hi,
I can't remember if there's a specially reserved area - but there is a series of lay-byes just outside the dock - but I'd guess they will be filled with trucks.
An alternative is a 30 minute drive north-east on the A16/E40 - the Texaco service station at Mannekensveere - about 1/2 way between junction 3 and 4 on the E40 in Belgium. This is a new service station (well a few years old now) and has nice wide bays for filling up and a few caravan bays.
Failing that there is a Belgian "Aire" between junction 5 and 6, and a large motorway service station at Jabbeke (note you need to follow the slip road to access the service station).
If you use viamichelin.com - you should be able to see the service station and the aire, and their closeness to Dunkerque.
I nearly always stop at the Texaco station on my commute - it's got OK toilets (showers too), coffee machine and reasonably priced fuel
Robert