2003 Toyota Landcruiser

Since the 80,000km service the car has been producing a LPG gas smell which seems to come into the cabin via the vents. We notice the smell after about and hour of continuous driving. When you drive the car for longer periods the smell becomes so bad you have to have all the windows open. It doesn't seem to matter if the vents are open or closed or if the fan for heating or cooling is on or off, the smell still builds up inside the cabin. I have taken the car back to the Toyota service centre 3 times. They have flushed out the a/con and changed the oils but still the smell persists. The gas smell cannot be smelt when you lift the bonnet and sniff around the engine. Please help, I am concerned for my family's health!
N.B It is a gas smell not a petrol smell.