I came to the same conclusion as HiJoeSilver and I too am about to close up the belly pan with gray only plumbing. It all came down to the fact that 3" hoses are readily available and fit right up to dump stations. It is really hard to find 1.5" hose of any type, especially one that has a smooth interior and is flexible/compact.
So all my plumbing is 1.5" until after the gate valve where it curves 90* and turns into a 3" connection. It's hard to explain but I recently posted some pics on instagram (lifeisriveting)
Follow our shell-off renovation on Instagram @LifeIsRiveting