We've slept there many times now, and never had a problem - but I do know other people are very cautious about staying overnight. We've not heard of any problems at Baie de la Somme, but as you say, it is always a potential risk - and you have to weigh up those risks. We only do it on our first night, because we arrive in France too late to find a campsite. On any subsequent 'travelling' nights we always make sure we find a campsite, and even in peak season when we used to go, have never had a problem finding a site which would take us for an overnight stop. In my experience the larger campsites may exclude overnights in peak season, but the smaller municipal sites are usually full of other people overnighting too.
The local gendarmerie do actually patrol Baie de la Somme about half a dozen times during the night and whenever we've stayed there we've usually been in the caravan area, which has been almost full. I don't think we'd stay if no-one else was around. On one occasion had to sleep near to the main building because there was no space in the caravan/motorhome bays.
If you do choose to stay overnight then obviously you would take sensible safety precautions with both car and caravan, including not leaving valuables anywhere in sight. We make sure that anything valuable is tucked away in one of the many secure and hidden places inside the caravan. Generally when you hear stories about robberies you also hear that the people concerned had emptied their pockets and left everything, including car keys, wallets and handbags on the worktop or the table!! Our car is alarmed - and we have a security device which we're certain would wake us up if there was an attempted break-in.
I certainly wouldn't suggest that Russell stays on an Aire in the south of the Rhone Valley - but if he does choose to then far better to ask advice from other people who have done it - and who perhaps can recommend an Aire where people feel reasonably secure.