Rachel Buchanan, from Oxford, was left with a "frightening" drive after floodwaters caught her by surprise.
The missing person is a 70-year-old German man who is feared to have been swept away by floodwaters while inside his caravan, police said.
Rachel Buchanan, from Oxford in the United Kingdom, was camping close to a river in Ardèche when flooding hit.
About 750 people in total - including many British holidaymakers - were evacuated in Gard, mainly from campsites, district officials reported.
According to the interior ministry, a total of 1,600 people, majority campers, were evacuated as a precaution in Gard and the nearby Ardeche and Drome regions.
"The level and speed of the river..."
"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".
However, Dousset, the top aide of the Gard administration, said no one knew for certain yet that the man was in his van at the time.
The Gard government said four German children were taken to hospital on Thursday suffering from hypothermia in Bagnols-sur-Cèze.
The dramatic flooding comes after a record-breaking spell of hot weather in Europe, where temperatures reached highs of 47C in some countries.
Several parts of central and southeast France are on flood alert after a blazing heatwave ended abruptly in storms on Thursday.
Numerous roads in the affected area remained cut off as night fell.
"There are still storms around, although they are starting to ease a little bit".
Meanwhile firefighters have been battling wildfires in Spain, Portugal and even within the Arctic circle in Sweden.