Really sorry to hear this. I never camped in France but was at a few rallies where everyone camped. Never heard of any problems. At Hotels they almost ALWAYS had a Bodega or some sort of inner court yards. I felt safe enough ... but still put on my disc lock and locked the steering. So simple.
Very bold move to steal bikes next to tents. Such is the downside to ear plugs when camping. I often use them because at rallies or group sites noisy party goer's can keep going till 4 am. No way to sleep through that ... Ear plugs in, Sorted!
BUT ... you'd never hear anyone making off with your bike either, so there's the trade off.
We've had similar happen many times at campgrounds in places I ride dirt bikes. Mojave Desert. Thousand show up there on big holiday weekends. They come in big motorhomes and caravans. They all circle the wagons like settlers of old, 10 or 15 rigs will group up, big White Man fire, steaks on the Barbie .. as if all was well in Right Wing America.
Thieves quietly walk in at 3am and take their pick. Nothing locked up. Lots of kid's mini's get stolen and some very high end bikes too. Same happened in the Sierra at big Moto riding area campgrounds.
The cops and Rangers are well on to this now and set up road blocks (usually only ONE ROAD IN/OUT) and check VIN's/registration ... they've caught quite a few this way.
Still, at a public camp ground or big event ... I put my Disc lock on just for luck. Takes all of 30 seconds to lock up your bike.
Would it surprise anyone if the thieves turned out to be from the UK?
Since bike theft seems to be a key national industry there?
And no customs check going back to UK. Easy Peasy.