The cost of carbon monoxide detector is so little that it's a no brainer not having one in a van. Those most at risk are the vans with the old type fire that vent through the roof as these fire are not balance flue and therefore not room sealed. I believe the new ones, such as the Truma S type, are now all room sealed and therefore present no hazard. However better safe than sorry and fit a C/O detector.
Just because an appliances is room sealed dosn,t mean its safe.
i am a gas engineer and can confirm that the appliance needs regular servicing and when the seals break down its no longer "Room Sealed"