Help me identify my Leak! I'm a newby.

[Sissy]

I have an 2008 3295rk (I have had it about 3 weeks) there is a leak I think from the black tank...! No smell..but dirty blackish water.

Haven't pulled the underbelly off yet. But there is a smaller plumbing pipe than the largest that then T's into the larger 3" drain pipes. If this is to the black tank, why the smaller pipe for black and the larger for the grey? Can someone identify what this pipe is? Does anyone know if black tank drains from the bottom of the tank or on the side?

Would this be a vent pipe to roof?
And my cap is leaking?

I'm stumped.