Hi Colin,
I have never been to the campsite you mention but I've heard it's good. The area is great. The weather is almost as good as that of the Cote D'Azur, so a good beach holiday in the main summer season is usually assured. The whole of the west coast from St Nazaire south to the Gironde estuary is beach almost all the way, mostly backed by pine forests which are great for cycling - full of cycle tracks so well worth taking bikes or hiring when you get there.
St Jean de Monts is a super resort. The streets leading off from the esplanade are full of small tourist shops but you have to travel further inland to reach the main town centre and market.
There is lots to do such as sailing, windsurfing, quadbiking, horseriding etc etc. There is a great waterpark called the Atlantic Toboggan which is well worth a visit and if you play golf there is a fantastic golf course.
A visit to the parc "Puy Du Fou" is a must (www.puydufou.com/uk)but make sure you give it a full day otherwise you wont get round everything.
There is so much do do and see in the area you will not be disapointed. Have a great holiday.