It can be hard to tell where it is coming from if it is high pitched. First thing to check is the Carbon Monoxide alarm. Find the box and see if it the indicator is red. They have a limited life and expire with the alarm on. Check the expiration date on it. Unscrew and unplug it. Same with the smoke alarm and propane detector.
33' 2008 National Tropical on a Freightliner chassis.RVM#189