Our rig has 2 grey tanks and valves for those tanks. One tank is for the kitchen sink, the other is for the shower and bathroom sink. Whenever I empty my black tank, I make sure to fill up both grey tanks first to get the optimum amount of flush. Now, why 2 tanks? When we bought our unit we were told we had a 45 gal grey tank, and we actually don't: instead we have a 20gal galley tank and a 25 gal bathroom tank. The reason is that in many national forests it is OK to dump the bathroom tank in the great outdoors, while dumping the galley tank is NOT allowed because of the food stuffs / particles from the kitchen which might attract wildlife, bugs, etc.