The pipes are all the same size. I found the best way to discover which was which was by adding a clear fitting. Now I know which valve is black and gray. You need a clear fitting anyway so you can tell when your black tank is empty and clean.
07 Dodge 1500 crew cab with 20" wheels, 08 Forest River Rockwood Signature Ultra-lite 8280SS
"The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore, all progress depends on the unreasonable man."
George Bernard Shaw