Hi Jon
My wife and I have been stopping overnight at autoroute aires in France and more recently Belgium, Switzerland and Italy every summer for the past 10 years or so. French aires vary widely from picnic areas with toilets right through to fully featured sites similar to UK Motorway services. They are much more frequent than UK services so if we don't like the look of one it is usually only about 10 miles to the next one.
We use our commonsense and only stop where there are other 'vanners already stopped in a good numbers and with good lighting, etc. We have a cheap Lidl door alarm lock set which goes off if the door is opened whilst we are sleeping. We keep our valuables and documents well hidden about our persons, in bed!!!, and in other places around the 'van. I am a generally VERY cautious person and choose my aires carefully. Especially on the French autoroutes aires are very frequent and so if we don't like the look of one aire we just move on to another.
Why do we do this?? The big advantage of staying at aires is that on long journeys we can drive until we are beginning to feel tired and then just pull over, drop the 'van legs, eat and go to bed. No need for unhitching and hitching and we can be away bright and early in the morning. We have sometimes kept going until 9.00pm and been away in the morning a 7.00am well refreshed. It is unnecessary to bother looking for a campsite or worry about reaching a site by a certain time. We do not have to leave the autoroute. The 'van remains hooked-up so no messing around setting up when we are tired. Just legs down in the evening and legs up in the morning. We can maximise our time travelling to our final destination. Oh and of course it is free, even on the toll-free autoroutes!! unlike rip-off UK motorway services.
On the practical side wehave the caravan set up to stop in this way. The bed is made up and we keep a little water in the onboard tank for drinking and a little water in the toilet flush tank so that we can use these facilities. We, also, use the fridge and cooker on gas whilst stopped.
I'm sure some people have been robbed at overnighting at aires and many people would call us foolish but we and many others have been doing this for years. We have met many other 'vanners, from all over Europe, overnighting on aires and had some interesting chats
We do sometimes stop at campsites near the motorways too. This is a bit more feasible as we have recently bought a sat nav which would help us to find stop-over campsites. We have POIs for Continental sites on the Sat Nav and there are, also, many site guides, eg, the Caravan Club's "Caravan Europe" which detail sites near autoroutes.
Anyway this is how we see things. I hope this is some help. However you decide to break your journey I'm sure you'll have a super time. We love Continental caravanning and go every summer.
Best wishes.
Tim