Hello Martin.
We have been to the area many times but never in August and never to that site.
Friends who had stayed there warned us of the shadow problems connected with being in the bottom of the gorge and on several occasions I have read of forum members experiencing problems with soft ground.
So as we particularly like staying in lakeside locations, we always stay at Les Salles sur Verdon which is situated at the other end of the gorge. See .
http://www.campinglespins.com
This is a lovely site with friendly owners the only problem of which is that in common with large parts of southern France, the Mistral can blow round the clock for days on end - if you have an awning it would need to be well strapped down.
On a slightly different issue, if you do go to Castellane take a look at P5 of this section where you will see comments on the Route Napoleon which is the road you would use to approach from the north.
It's a glorious drive but .
The comments will speak for themselves..!
Hope this helps,
Olly.