The two gray tanks and the black tank only join at the sewer outlet, or just out of sight of it under the belly cover. There is no direct cross-over between tanks, although some folks have done just that by opening both valves and using the sewer discharge plumbing (valved and capped) to do just that. Gray #1 has a smaller discharge line than Gray #2, also, so when you dump it, it will seem to be slow.