Les Rivages, as mentioned, is a site I would have normally highly recommended. However, according to friends we met a week ago, who had just come from Les Rivages, there has been some sort of noisy excavation going on just opposite the site, which starts around 7.30 in the morning and continues all day - with noisy pile-driving, and coming and goings of many site vehicles. It's a huge job, apparently, and is expected to last for the whole summer. They've stayed there lots of time before, but did say that if this had been their first time it's not a site they would go back to because of the noise which they said was seriously unpleasant.
However, there are lots of other sites in Millau at the other side of the bridge which are unaffected by the noise from the excavations.
Les Peupliers is a lovely site, but access can be a little tight for a large outfit. We've no experience of Les Caussanel, but it isn't in the most attractive part of that area - Millau is at the centre of the Gorges du Tarn/Dourbie/Jonte and by far the best place for a short stay. Personally I'd have a look at some of the other sites in Millau and choose one of those. There are three sites together, Les Rivages, Le Cote Sud, and Les Deux Rivieres all on Avenue de l'Aigoual - probably these should be avoided, but the others on Avenue de Millau Plage are probably unaffected.