Window air conditioner with extension cord safe?

[Tricia O'Brien]

Is is a big safety hazard to plug a 5000 BTU air conditioner into a heavy duty 6 ft or 12 ft extension cord and then plug that extension cord into the wall?  I know the manufacturers do not recommend this, but I was wondering if anyone has done this and did it work out well for you?

 I have an Indiana rental home where the central air conditioner just  quit working and I have decided to give the tenants some window A/C units instead of replacing the old one due to cost.  They prefer 3 small units (one for each bedroom) rather than a large unit in the living room and one in the kitchen. I just discovered one of the bedrooms does not have an electric outlet next to the window.  So I'm trying to figure out a solution, and I think it would take at least 2-4 weeks to find an electrician to do a small job like adding one electric outlet to this room.  Plus might be several hundred dollars. Thanks for any input!