Over Night Sop in France

Hi,

I'm going over to France at the end of July, on the Euro Tunnel to Calais then driving down to near St Malo. We plan to take an overnight stop somewhere near Abbeville.

My question is, how easy is it to find somewhere when on route for a stop or should I book a site before I go?

It would be nice not to book so if we wanted to travel a litle further before stopping we can but am worried we will get over there and not be able to find a site to stop at.

Your help, past experiences appreciated. With thanks.