My Eagle has two grey tanks, a 50 gal for the shower and a 30 gal for the galley. The galley tank is located under the kitchen sink, above the floor. Because of this it works well for flushing the black tank. After emptying the black tank I leave it open and close off the outlet with an auxiliary valve, then open the galley tank. This allows the galley water to backfill into the black tank. Then by opening the aux. valve I get a good backflush of the black tank.
2012 321RLTS Eagle
2011 F350 CC SB