The Aire de la Baie de Somme on the A16 autoroute is a good place to stop overnight. I doubt you will find any campsites accepting arrivals after midnight. We've stayed there a lot and there is a dedicated caravans/motorhomes area in a cul de sac at the far end of the area - follow the caravans sign. There are other caravan bays in various other parts of the 'aire'.
It's a lovely place - with canals full of carp, ducks and other water birds, and there's a viewing tower where you can see over the Somme Estuary. There are free showers in the morning too - but perhaps only in the men's section (but I use it anyway)!