From the French official website for caravanes and motor homes :
You can stay in a private property (with the owner permission) for a total of less than 3 months per year.
If you want to stay more than 3 months, the owner has to ask the local mayor for the legal authorisation. (Google translation)
"Camp and park on private land
The parking of a leisure vehicle on private land depends on the Town Planning Code. A caravan or motorhome can be freely stored in a private garden, without requesting authorization, pending its next use.
An individual can camp in the garden of another, with his agreement, and not more than three months per year (consecutive or not) and in accordance with the local town planning document.
For a final installation of a caravan or a motorhome in your garden, you must request an authorization from the town hall: a prior declaration for an area between 5 m² and 20 m², a building permit beyond."
Tout savoir pour camper et stationner en France : stationnement d'un camping-car sur un terrain privé, stationnement d'une caravane sur la voie publique...
ffcc.fr