Some things are relative and particular to your trailer and your setup.
When our trailer is level we can flush and wash with about 5 or ten gallons in the tank.
When it is on the truck, however, it is about two inches nose up. It tracks fine and the axle loadings are close enough to not worry about. The pickup tube for the pump is somehow located where that small angle is enough to make the tank unusable with less than about 15 gallons in it.
We start out with a little over 20 gallons to take care of potty stops along the way. Since we don't boondock we are on campground water once we set up.