We've stayed at Les Rivages a few times now - usually in low season, when it takes an ACSI card, and costs just 15 euros per night, all-in. We had a riverside pitch, with a view of the Millau viaduct. Pitches are generally a good size, grassy, some with hedges between, others more open, the sanitary blocks are fine, there are a couple of swimming pools, and a restaurant which did very good value 'Menus du Jour' with 3 course for 9 euros when we were last there a couple of years ago. It's walking distance into Millau centre, and there are always paragliders flying over the site, which is spectacular when a lot of them are airbourne!
We also enjoyed it - in low season it's much more low-key and quiet, with no activities, and the shop isn't open - but there are many shops in Millau centre, and three large supermarkets on the outskirts, all selling cheap fuel!
We may be back this year! If I could post a picture on here of the site then I would so do (I've downloaded the instructions but haven't studied them yet).