I don't know about the port itself - it's years since we went via Boulogne - but you could stay overnight at the Aire Des Deux Caps, on the autoroute between Calais and Boulogne, or if you're heading the other way, the Aire de la Baie de Somme near Abbeville (which is where we'll be on a Friday night in late August after a late arrival in France. It has a caravan/motorhome area, separate from the main service building, with a chicane entrance so that HGV's don't get access. It's a very pleasant place to stay too, not noisy in the caravan area, and a lovely walk on a boarded walkway raised high over small dykes full of ducks and carp, back to the main service building.