What Darylkalbach stated is correct.
If you don't have a clothes washer installed, or not using one at the time, you can extend the galley/shower gray tank by interconnecting it with the vanity/washer tank. To do this, purchase a Valterra valve or similar to this one and attach to your sewer hose connect point:
By closing this add-on valve and opening both of your gray tank valves you can use essentially all of the 80 gallons of gray water capacity.
When ready to dump tanks first close the two gray tank valves. Connect hose and open the add-on valve which will drain gray water from the two gray water tank pipes. Then, dump black tank and flush, followed by dumping both gray tanks.
Hope this helps.