On our 3725, our galley tank is about 6 inches or so higher up in the frame than the black tank. I installed a shutoff valve on the sewer outlet so I could divert water around in the tank systems. I dump the black tank first, while running the flush system. After that, I close the shutoff valve at the outlet, leave the black tank valve open, and open the galley tank (tank being full) and let it backwash into the black tank. Then I open the end of line valve again and let it go. Surprising how much more comes out that way. Final hose washout is done with the bath/shower tank water.
