Vents on roof

[mtlakejim]

In the attached picture you can see we have 4 black pipes/vents on the roof plus one flat white one.

Based on the location I am hoping that the one black vent that I added the finned venturi to is the black tank vent (it is in the wall directly behind the commode and the commode sits directly over the tank).

However there are three more vents forward and to the right of that location all together. I am guessing that there is a vent on each of the two gray tanks but what would the third one be for??

Then there is that flat white vent towards the rear of the unit??