I am no expert but I believe an increase in carbon monoxide can make you sleepy or fall asleep - which is what catches people out. Remember that the small camping heaters are designed for tents where there is plenty of airflow, a van will be more airtight so no airflow.
But we all make cups of tea in our vans and have the hob on for 10-15 mins so perhaps you could boil a kettle and a pan of water and turn the hob off and the hot water filled kettle will act like a small radiator . Not sure how long it will take for the van to cool down though! I do notice the air quality change in the van (probably other fumes) when I have boiled a kettle so I tend to open the window but I do use the van in the winter just early spring to late autumn.
End if the day there is risk in everything we do - using the hob to heat your van with all precautions just increases your risk of dying form carbon monoxide. Bit like Russian roulette you might get away with it but there is a high chance you might not!
Just my thoughts.