No-one can be certain that anywhere is 100%, but for more the past twelve years we have regularly overnighted on French (and German) motorway services with our (now) teenage son. Many of them have dedicated areas for campervans. It is worth noting that the French toll roads have video on, video off cameras, offering a degree of security and that, as the receipts often make clear, paying the toll "buys" you 24 hours on that road. There are frequently picnic areas as well as shops and toilet facilities, which may be useful. Common sense, of course, should prevail, and I would not leave a van unattended and out of sight for long. BTW, the stops on non-toll trunk roads frequently seem more vulnerable (and less hygienic!)