Have been mulling this over. 'Up market' and 'campsite' are not words which necessarily go together .
However, I do remember talking to a camp rep in the south of France and commenting that the mobiles were in very good condition.
She said this was because the sort of people who go to the south of France are the sort of people who take care of things - i.e. you get a better class of person going to the south of France than you do elsewhere. That was her theory, anyway.
Campsites in the south of France have more French in them than other sites a) because the French don't do abroad as much as other nationalities and b) they like the sun as much as the next nationality.
If your friend is going in the school summer holidays, she'll get good weather - she may well roast in a mobile. (We have, in the past.) Although it is more expensive, she may want to think about aircon.
I must say, I think that in the main, it's a certain type of person who likes camping (tent or mobile) holidays...professional, family-orientated folk, so not likely to be chavs, if that is her concern.