I removed the front furnace (interior and housing) and capped the LP and the smell is almost gone!
Now I just don't know whether it is the furnace or the LP fitting to the furnace. I did spray soap and water on the fitting so my guess is the old Duo-Therm furnace has 'issues'.
My wife gets a big assist since her sinus system is far more sensative than mine. She actually smelled gas some time ago when we were living in Jellybean III as temporary housing.
Now I need to consider fixing the furnace or replacing it.