Black/Grey tanks

My 100 year old grandmother brags to this day about the ingenuity of my grandfather who installed electrics and a flush toilet in their first camper. He cut a hole through the floor and bolted down a porcelain throne. He'd just fill up the water tank when they needed it, and gravity did the rest. That's right, no black tank, just a hole in the floor.

Ahh, the good old days!

As was mentioned above, you get more options with separate tanks.

Our 1981 truck camper came with just a single smallish black tank. You can leave the black tank main knife valve closed and the toilet flushes into the tank. But the grey water can by-pass that knife valve to the end cap that has a small capped hose connection in the big cap.

So you can either:

A put a hose on the outside cap to drain the grey water away somewhere, or
B. Leave the outside caps on, and open the tank knife valve so now the grey and black both fill the one tank.

(At the sani-dump be sure to close the knife valve before taking the big cap or little cap off! :) And have the sewer hose ready to catch the initial flow from the small hose connection before you take the big cap off.

I got a second black tank from an old camper (garage sale) to be the grey tank on ours and did some plumbing. ( With the 11 footer it hangs out the back of the long bed truck enough so there is room for the second tank.)

With a home septic system you have a leach field for your tank to continually drain into. A holding tank on a RV is just that, a holding tank and not a septic system.

There are still some (albeit relatively few) places where it is legal and reasonable to dump/dribble gray water on the ground or into a dry well, but not black water.

When connected to a full hookup site, it's sometimes convenient to leave the gray water partly or entirely open and not have to worry about how full that tank is. This should not be done with the black tank since it would tend to leave the solids piling up without sufficient liquid to have them rinse away as desired. A house doesn't have this problem partly because a lot more water is used in flushing household toilets than RV toilets (and also partly because septic tanks operate on different principles than holding tanks).

Finally, it may have something to do with people not really liking black water overflowing into their shower pan when the tank fills up.

doughere wrote:
My unit originally had 40 gal grey and 40 gal black. 40 gallons of grey is good for 4 days, 40 gallons of black for 10 days. Not very sensible.

Of course it's sensible. With just my wife & I using our camper and just the toilet feeding the 30 gal black water tank we could camp for at least two weeks straight without having to give any thought to off loading the contents of the black water tank. The 30 gal grey water tank I typically off load every day or two and because it's MUCH easier to deal with grey than black no way would I want a combined black & grey tank.

You have to go back PRIOR to 1978 to see any RV that had 1 Waste tank(There were some, very few, that had single tanks after 1978) . So, Is your question relevant after almost 40 years? Probably not. Weight distribution is not a factor. The plain fact is, most people want 2 tanks. You will put more waste in the Gray tank than the black tank. So dumping the Gray just makes more sense and you can go longer if you do not fill a single tank faster due to all gray and black going to a single tank. AND it gives the Black tank chemicals time to completely dissolve the Black waste. Doug

RVs use to have just one waste tank......black/grey all went to that one tank
(Small campers/truck campers dumped grey---just a black for toilet)

THEN folks wanted bigger.....more bells/whistles

Space limited so separate tanks

Then more, more........
Space still limited so more tanks added (black/grey/galley)

Then more, more........
So split baths------double black/double grey/galley

There is only so much room for tanks

I have black/grey/galley that all tie into to a common drain header so I added an extra dump valve on that header. (there is a twist on version)
With it closed and all tank valves open I have 128 gallons of waste capacity

Yep.....just like a residential waste system. All together.
They are WASTE tanks. Grey just as nasty/smelly as black----worst due to food debris, grease/oils, hair, sloughed skin, body oils etc.

I run out of fresh water before filling waste ---good for 7 days

...this possibly a holdover from when many units had a black tank, but were designed to essentially dump the grey water on the ground?
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Yes, in the past. Now it's more likely it's a weight distribution thing.

I personally think it's better to separate, especially if the tanks are small. I don't mind hauling grey water in a tote to the dump, rather not tote black. And like you my septic is all in one at home, but irrelevant.