The photo below is what Emmel is refering to---
Keep this valve closed and the shower and galley tank valves closed. As water will seek it's own level you are now sharing the 2 tanks. OF COURSE, you will also keep the black tank closed.
When you are ready to dump...[list_to_fix][*]Close Gally Valve [*]Close Shower Valve [*]Open Black Tank [*]Open extra valve pictured above.
during this segment you can also flush the Black Valve.[*]Close the Black Tank [*]Open Gally Valve and Dump
When Gally has dumped Close Gally Valve[*]Open the Shower Valve
After Dumping of all 3 tanks, Leave the Black Tank Valve closed and the Extra valve closed. Leave the Shower and gally valves open