The gray tank - black tank configuration came into being sometime in the middle of RV history, but it is certainly not the only configuration ever used, and not the only configuration used today.
Many early RVs had a single tank, some for everything, some for the toilet only, letting gray water drain to the ground immediately. Many tent campers today are still configured this way, black tank only, whether portable or permanent. If you can't drain gray to the ground, you have to catch it in something.
Gray tanks became common practice when bathtubs and showers started becoming RV features. A shower usually spills too much water for it all to go to the ground at once. A gray tank lets you either hold it, or let it dribble on the campsite through a slightly open valve.
As fewer and fewer places permit you to dribble gray water, the value of having a separate gray is lost. We could see RVs soon going back to that single large tank configuration, which has not entirely disappeared.
Many large commercial travel coaches use a single large holding tank for all liquid waste. This gets past the problem that one of the tanks fills up long before the other, depending how the facilities are used. There is usually someplace under the floor with room for one very big tank, the space is heated and there is room to run plumbing to the tank. No need to build separate tanks immediately under kitchen, toilets, showers as we see in RVs.
Some smaller RVs, B's could be included, are also easier to fit with a single waste tank than with two separate tanks each of adequate size. Or some really old ones might be like the pop-ups, the gray just goes right to the ground.
At the other end of the scale, some large RVs today might have three or more holding tanks, depending on how many bathrooms and where they are located relative to the kitchen. There might be two grays and a black, two blacks and a gray, or even two of each. It is easier to put a small tank under each point of water use than try to drain to something remote. Downside is that plumbing for dump locations and dump valves gets complicated, RV users have to learn complex dump procedures like "empty this one first then empty the one on the other side."
Tom Test
Itasca Spirit 29B
