The man, who is believed to be in his 70s, was helping supervise over 119 German children and youths who were camping on the German-owned land in Saint-Julien-de-Peyrolas. The area is located in the Gard region, around 60 kilometers (37 miles) north of Avignon.
Rescuers believe the man sought shelter in his camper when fast-flowing water burst the banks of the nearby L'Ardeche River, but the vehicle was swept away.
The heavily damaged camper was found near the river, but the man was not inside.
Enquiry openedOn Friday, the German president and vice president of the Leverkusen association which owns the land on which the camping site was located were taken into police custody.
A police enquiry has been opened into the case concerning "unintentional injury, endangering the lives of others, illegal employment and running a camp site without authorization," Eric Maurel, the Nimes prosecutor is reported to have said.
The French Interior Ministry said 1,600 people were evacuated from the region, including 750 people from five campsites.
Four of the rescued German children were taken to the hospital suffering from hypothermia, officials said. All in the group, who are from Leverkusen, are accounted for and safe.
A total of 10 people in the region were hospitalized for minor injuries due to the flash floods.
Massive rescueAfter weeks of dry and hot conditions, recent torrential downpours quickly swelled the Ceze and L'Ardeche rivers in the Gard region.
Muddy water swept through the region, taking many foreign tourists and other vacationers by surprise.
Hundreds of firefighters and police took part in the rescue operations, using helicopters to help spot affected campsites and evacuate people.
rs,jm/sms (AP, AFP, dpa, Reuters)
Each evening at 1830 UTC, DW's editors send out a selection of the day's hard news and quality feature journalism. You can sign up to receive it directly here.