The badly decomposed remains of a middle-aged man were found in a white Bedford van parked in a corner of the Gordano Services on the M5 near Bristol.
A passing woman motorist raised the alarm after she spotted the man's body inside the vehicle at about 8am on Saturday.
Avon and Somerset police are awaiting the results of a post- mortem examination but believe the man died of natural causes.
Employees at the service station - a popular break for holidaymakers heading for the West Country from London and the Midlands - said the van had arrived in the car park more than three weeks ago.
David McCann, operations manager, said police had yet to confirm how long the van had been in the car park. 'If it was there for several weeks then we will look into why it was not reported earlier.'