Graham,
Like anywhere all over Europe you have to be sensible and cautious, I always apply the rule of parking with other vehicles. Unfortunately France is probably no worst than other countries. We as caravaners just hear about the issues more because we visit there far more than other countries.
Generally I don't believe that parking with a Service station in sight will help (based on the last 3 reports I've had of people being gassed in Service areas)
I could tell you load of stories of Albanians robbing people at knife point in Italy, and people being robbed by gypsies in Spain but it would put you and everyone off having a nice holiday.
Rather to Evans of Spot and others what you need is the combined gas detectors which detect Assault gases as well as normal (cooking/heating) gases. Secondly securing doors properly, fitting a small safe in a Bed locker. Fitting the old fashioned but effective rape alarms with string to the door is an effective and cheap fix.
I know it's easy to be overtaken with the holiday spirit and let caution to the wind, but this could result in a memorable holiday for the wrong reasons
Regards Monkeys husband