We have just returned from a 2 week camping holiday in Olonne sur mer. I found most people in the area mainly French speaking. The campsite was approx 2 miles outside Sable D'Olonne, this had adequate facilities inc a bar/restaurant area open from early morning till late evening, a shop with fresh bread, milk etc. daily, indoor and outdoor pools, the beach and surfing were fantastic and clean but 2km walk. The campers were a mixture of families and young people on the site, with tents, touring caravans, motor homes and static caravan homes. Staff in the bar spoke English, but toilet paper and toilet seats were hard to find in most WC's, mens showers were ok but womens had low water pressure, the nearest supermarket called 'Super U' is in Olonne sur mer, where beer is cheap e.g. 40 X 30cl bottles of Kronenbourg 11 euros (£8)... the weather was excellent 24C most days, very hot for us... mosquito bites are minimal but unavoidable, the location is near salt marshes which probably attracts them. We took our bikes and there is canoing 3 miles away, gokarting 5m, the main town Sable D'Olonne is great, with thousands of French on holiday in hundreds of restaurants along the sea front, Moules et frites (mussels and chips) are the local dish. La Rochelle is another beautiful own over an hours drive away, and Pou de Fer between La Roche sur Yon and Clochet is a good family day out, similar to a theme park only with live open air shows of vikings, knights and gladiators instead of rides.
Overall a nice holiday with fantastic weather and a challenge to order food in French.