Yeah, it is rubbish! One of those urban myths that goes around because people can't accept that someone can break into your caravan and not wake you up. Basically the 'stories' are only ever "we think someone gassed the caravan/motorhome etc." but in reality there is no actual evidence.
The truth of the matter is that the amount of gas required to guarantee to knock out everyone in something the size of a caravan/motorhome is far more than is practically possible to transport, not to mention too expensive for it to be worthwhile. Even if it was possible/happening, there would have been many deaths from this type of attack - anesthetising someone is a science, not a matter of sticking a hose in a vent!
Trust me - if there were any chance of this being a reality, I would be the first one in line spending my money on one of those detectors!! Personally I'd prefer to spend the same amount of money on a carbon monoxide detector (rather more important IMHO) and a decent secondary lock for the door.
I have slept in the aires myself in the car/van several times with no issues, but I have to admit that when it was one of the more remote ones, I was a bit weary. Most people seem to pick the ones on the autoroutes and park right next to the service station where there are always plenty of people about and it is well lit.
Matt