A 70-year-old German man helping to supervise children at a summer camp at Saint-Julien-de-Peyrolas in Gard was feared to have been swept away by floodwaters along with his van yesterday.
Top Gard official Thierry Dousset told BFM-TV that about 750 people were evacuated from five campsites.
The flash flooding came about after a hot spell in France, and the Ceze and L'Ardeche rivers quickly burst their banks.
Police said the caravan was later found "empty and in pieces" after being swept away by water that overflowed from the Ardeche River.
Ten people were taken to hospital suffering from hypothermia and minor injuries, said regional police spokesman Jerome Tallaron.
A fellow camper told France's BFMTV channel that as the river surged through the site, he could hear children's cries in the trees.
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'Driving in our camper van was very frightening as the road by the river was completely under water and waterfalls had appeared from nowhere, crashing down the gorge'.
He was the only person unaccounted for after a rescue operation at three campsites in the area, where a heatwave ended abruptly this week in storms that caused flash flooding.
Roads remained close throughout these Gard, Ardeche and Drome, but will be reopening as the water level gets lower.
Authorities warned that the flooding would take time to recede and urged people to be vigilant.
Several parts of central and southeast France are on flood alert after weeks of blazing temperatures erupted in storms on Thursday, turning several campsites into mudbaths.
Heavy rain meant tents were destroyed, while about 17,000 homes were without power.