Pont d'Avignon is a Camping Cheques site (www.campingcheque.co.uk) which takes Camping Cheques up until 4th July, indicating that any time up until then you should be able to get a pitch without advance booking. Fontisson and Durance Luberon are both ACSI Card sites - Fontisson at 13 euros, and Luberon at 15 euros (www.campingcard.com). I know as Brits we're used to having to book everywhere well in advance, but it just isn't like that in France - they're much more geared up to people just turning up.
We always go in late May/early to late June - and have never booked anywhere. We use Camping Cheques, and an ACSI card, both of which can give you really good discounts on the normal campsite tariffs.