If I understand your description. The grey water
lines flow into a grey water
tank from there to a thru-hull and overboard
and maybe to a pumpout deck
I'm guessing that the output on the tank is at the bottom of one of the sides of the tank. You turn a valve and the grey water flows overboard
(I'll leave the pumpout out of the discussion now.)
If that's the case, it's always best to have a straight run. Otherwise you have standing waste in the line.