The 38-year-old man and his 34-year-old girlfriend were spending the weekend near Mörlialp in Giswil, Obwalden, reported the Luzerner Zeitung this week.
Investigations suggest the pair decided to heat their campervan during the cold night by placing hot coals in a pan inside the van.
With the windows closed, the van quickly filled with carbon monoxide which killed the pair while they slept, according to the prosecutor.
Police have found no evidence of criminality and are treating the case as a tragic accident.
The bodies of the couple were found by police on January 24th after the pair failed to reappear at work that week and were traced to the campground via their mobile phones, said the paper.