It's a common factory shortcut to do something like that. If its easier to run the drain from a sink or shower to black instead of gray, they may do it. Changing it after the coach is finished is often a challenge, simply to get at everything. The plumbing itself is easy, if you can see it and touch it.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC