Do you have any manual(s) for your Atwood furnace? Of no, then I would suggest determining the model of furnace you have and then click here.
This will take you to Atwood's website and you can see if there are installation manuals available for your furnace. In the manual, there will be a parts breakdown and you can see where the sail is located.
If you can't find anything on Atwood's website, Don't give up. Try googling the make (Atwood) along with the model name. You might hit on a site that has information on your problem and possibly a manual or documentation you can view/download. You might even find something on troubleshooting.