Holding Tank Vent Caps

Racerguy

Member
I have a Monticello. On the roof I have what appear to be 4 vent caps. 3 are identical. I am assuming that they are for Black, and grey 1&2, But there is a 4th cap that is near to what I believe is the holding tank vent. Both are just forward of the toilet ceiling vent. Can anyone tell me what that cap may be for?
 
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