Extra Internal Door Security Advice

I too was sceptical about gas attacks, but something that happened to some campers on the next pitch to me, on a campsite in France, makes me wonder.  I was woken in the night by the sound of doors closing.  It annoyed me as it was one on these "how many doors does that car have" scenario.  I did not look at the time.  The next morning there was a lot of toing and froing around the tent next to me.  The young couple from the Netherlands had been robbed.  Nobody else had heard the car doors (The nearest other unit was a few pitches away).  The tent had been slashed near where the campers heads were.  A larger slash was near where their kit was kept.  They lost a laptop, phones, clothes and all their cards.  Before they discovered that they had been robbed they had felt unwell.  The Gendarmes were very helpful, but said that tents were often targeted in this way.

I use town Aires, but not stop offs on motorways.