Holding tank vent covers

I just went up and replaced the three 1 1/2 '' holding tank vent covers. I also have a 5"" dia. vent cover that has a bolt end that it is screwed on to. I got the 3 covers at camping world but they did not have the 5" one. Can anyone tell me where I can get one or can I get it from Heartland Parts?


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Not sure where to get it, not sure what it would be exactly, but could it be a vent to let heat out of the roof/attic area, kind of like soffet (SP?) vents on a house?