The last PC out of OR the 1st PC into your bus will be a basic "blade valve" like this
http://www.amazon.com/Camco-39513-3-.../dp/B000EDUTQ8
This one doesn't have ends on it yet, you can have several of these upstream if you want even
Most people will put a 3inch outlet so they can use run of the mill sewer hoses and caps for rv's
The inside which hooks up to plumbing can be different sizes normally 3in or 4 in depending on what you use
Remember that almost every dump station and camp ground is made for a driver side dump, if only room you have is everything on passenger side that is okay if you have room to run solid pcv pipe at a slope down and across to passenger side, that is what I originally did, I am moving it this spring