Rod I would've thought you more than nearly anybody else would know that you're (I assume) referring to a carbon monoxide (CO) alarm and not a carbon dioxide (CO2) alarm
The smoke alarm Lunar fit as standard has a silencing mode that lasts 10 minutes after you've pressed the 'test' button when the thing is sounding. That's why you hear alarms on caravan sites that only last for two bleeps and then strangled silence. They don't beep mournfully, they just fume in silent frustration whilst conflagration occurs all around.
I've got a Kidde combined CO/Fire Alarm available, only 18 months old, one owner, used to living in a Troll, but prepared to downsize.
I certainly can't recommend the Fire Angel CO Alarm that Lunar fit as it has a habit of sounding at random 8 hours after any combustion took place anywhere near the van.