Ian, presuming you're the same Ian asking the question about overnight services, I'd suggest the Rouen route too - by far the best way to get to Royan from Calais - and you can stay at Baie de la Somme service area, too. See mappy.com for details. Don't go via Paris unless you absolutely have to - and if so travel on the E104/A104/N104 (whatever it's been renumbered) which is the outer/outer/outer Paris ring road - and actually fine, but you shouldn't need to go this way.
We stayed at Baie de la Somme in May this year - and it was lovely. We've overnighted there several times now, with no problems - although it does get busy. There is a dedicated caravan area in a cul-de-sac to stop HGV's getting in, but it fills up early. This year we stayed nearer the service station, but it was quiet, and we had a really good sleep, and an equally good free shower in the morning, plus a lovely breakfast of croissants, orange juice and coffee in the service area!!1