We use a combination of Camping Cheques, Touring Cheques (Eurocamp) and ACSI. We don't pre-book sites - the whole idea of these schemes is to fill up empty pitches, so if the site has decided on it's 'low season' then even if the site is 'pretty full' you're still able to get a pitch. We've been using Camping Cheques for 12 years, and we've never been 'given' a pitch whichever we use - just allowed to choose our own. I think it's actually an advantage to not book or have a pre-allocated pitch - you can then find a pitch which suits your requirements. The only time we're ever allocated a pitch is at Chateau du Gandspette, and this is more to do with the time you're planning to leave, or whether you're staying for just one night. I suppose with a large van you may need the added security of at least emailing to find out if they have a pitch big enough for you.
We've never previously found a bad site - but this year found one we really wouldn't want to stay on. This was an ACSI site -and it doesn't appear to be listed now, so perhaps others agree with us!