Manufacturers tell you not to use extension cords because they lose control of
the wire size. They know what their cord is, the plug type regulates what is
supposed to be inside the wall but an extension cord can be anything.
As long as you are using one of those "heavy appliance" cords, 14 guage or 12
guage, you are fine. I would suggest checking to be sure the plug isn't getting
hot occasionally since that is usually where these fail.