Clive,
It is most important that you DO NOT, under any circumstances, use the fridge on gas until you hav ehad it fixed.
On a 2004 van it will have th eold style mild steel burner which has rusted and is causing improper burning of the gas , which is producing the soot and smoke, and more deadly, LOTS of Carbon Monoxide.
DO NOT USE IT ON GAS.
I really cannot emphasise this too much.
It is also worth noting that Thetford fridges MUST HAVE a full burner service every year, in addition to the normal Annual Caravan Service.
Because Thetford fridges vent the exhaust inside the body of the van, it is YOUR LIFE that is at risk of ignoring the advice.