Why does having the furnace on produce a sewer smell ?

Big Love wrote:
are you sure it is a sewer smell and not an unburned propane gas smell?

I was thinking the same thing. On my first trailer which was used when I bought it, propane would leak from the in line gas shut off valve's stem. Would get a whiff of propane every time the furnace fired up. Found out where it was coming from. While trouble shooting I would smell the propane when I operated the shut off valve.