I知 trying to figure out the grey and black tan situatuion.
I see two valves under the fifth wheel. One forward nearly under bathroom. I知 assuming that is the black water tank. There is another black pipe nearly under Kitchen. I知 assuming that is for the grey water.
But, there is another white pvc pipe hanging between the two black ovc pipes. It is much smaller but still pvc. What is that for?
Your grey and blank tanks share the black 4" drain pipe. Drain black first, then grey water second. Rear black is for your kitchen grey water. Smaller white is your drain for fresh water on board tank.