An informational reminder since people new to the site may be unaware.
In the mid-1960s Airstream used furnaces made by International Oil Burner. These furnaces are unsafe and should not be operated, even for testing. They are known to have a gas valve that will not always close when the furnace shuts off, and also have a burner prone to internal structural failures that can lead to fire or explosion. Repair parts are unavailable; these furnaces should be scrapped.
From 1966-1977 Airstream used Suburban NT-17, NT-22, and NT-30 furnaces which are all affected by a recall.
The furnaces utilized a rubber crossover tube that would crack and fail over time leading to furnace exhaust being blown into the trailer interior. If there is no paperwork to document that the recall was performed, it is necessary to remove the furnace from the trailer to inspect it and find out. While parts are not available through official sources, some forum members have been successful in replacing the crossover tube using stock materials.
Mods may wish to make this a sticky at their discretion.