[flybouy]

There are no diagrames of anything from Keystone. Is this a new issue or is it the first time you're using the furnace? I'm guessing the furnace is under the the front bedroom steps. Typically there are return air grills in the step risers. Typical setup is a central aluminum duct running down the center of the trailer from front to back. You can remove that diffuser and take a look down to see if the duct is obstructed.