You'll find the cheapest fuel around Calais is at the Auchan hypermarket on Avenue Roger Salengro. (Check this website for details: http://www.zagaz.com/prix-carburant.php?id_div=35087). You can use the same website to find the lowest priced fuel in any location in France, and it's generally fairly up to date. If you 'Google' for 'Hypermarche, Calais' it shows you the locations of all the hypermarkets on a map (some don't sell fuel). You can zoom in on the map and find your route to the hypermarket from the port (basically follow the 'Centre Ville' signs just as you get out of the port.
You may find it difficult to get into the fuel stations in hypermarkets if you are towing - some of them have very narrow exits, with chicanes, which are impossible to wiggle through with a caravan.