Mine are screwed to the ceiling. I'm not sure about other type vents. As far as junk in the runs, read it on the forum. Also where the main cover is, you can lower it and make sure the air is going into the runs. Some have used duct tape to prevent air from by passing the opening on each side. Mine is Styrofoam runs with pieces loose in them.
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2012 SunSeeker 3100SS Toad-1962 Futura Average 100 + days camping
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