I know this has been discussed before but I wanted to be really clear on this issue/solution. This weekend we camped without hookups. Which is fine and 4 of us were able to take showers and the gray filled up but not over. We were careful to make sure of this. Another day and this would have been an issue. Since the black was only at 1/3 it occurred to me that I could extend the gray tank capacity if I opened both valves and "shared" the tanks. I know there is a more permanent way to do this but opening both valves would serve the same purpose right?Is this an Ok practice? Not sure why but I always felt weird about combining the 2 tanks. But I do know some higher end rigs only have one tank so it must be OK and a lot of you do this to help the gray flush the black when dumping. Is there any harm in letting my 2 tanks comingle? If not why do they even bother with 2 tanks instead of one?There are traps in every sink that feeds the gray, and even some bathroom sinks feed the black. So why not just have one big tank?

Can I watch when you go to dump? I would dump some grey down the toilet to balance, not open the valves and use the end cap for holding it all in.

I should have mentioned I have a macerator permanently installed so I never have to hook up a hose and don't have an end cap. I can see where doing this and dumping conventionally would be an yucky mess. But I don't ever need to touch my hoses so opening the valves is still a closed system unless I turn on the macerator. The macerator is essentially an automatic end cap. Matter of fact even when I have hookups I leave the gray open but it won't dump unless I hit the macerator switch. So this method wouldn't work with a conventional gravity drain but can't see why it wouldn't with my setup. The question is really about an issue with combining the tanks not actually dumping.

I always thought it was lower end rigs that had one tank (cost cutting) I would not do it! My grey tank will fill 4 times faster then the black, and their have been rare times when I've dumped grey water out side the norm, and would never do that with a mixed tank. I don't thing it's a good idea. Putting grey water in the blk tank ok, blk in grey, no way. Perhaps replumb it to transfer grey to blk tank.

You can use one of these. It attaches to your sewer output pipe just like your hose attaches, and becomes a third waste valve. Keep it closed and open your black and grey tank valves and you'll essentially have one, big waste tank. When it's time to dump, hook up your hose to the output side of the "third" waste valve and open it to dump. I have mine set up like that and works perfectly when needed. No permanent installation necessary. This one is made by Valterra. There are a couple of different brands available though. These are also handy of you have a waste valve that is leaking slightly. You can use one of these to keep the leak contained until time to dump and until you can get the leaking valve repaired/replaced.

mtrumpet wrote:You can use one of these. It attaches to your sewer output pipe just like your hose attaches, and becomes a third waste valve. Keep it closed and open your black and grey tank valves and you'll essentially have one, big waste tank. When it's time to dump, hook up your hose then open the "third" waste valve. I have mine set up like and works perfectly when needed. No permanent installation necessary. This one is made by Valterra. There are a couple of different brands available though.My macerator essentially serves the same purpose. It's closed until I turn it on. But I can see where this would accomplish the same thing on a conventional gravity dump.

Combining black and Gray | Good Sam Community

22 Replies

Old-Biscuit
Explorer III
May 11, 2015
Been making this 'mod' to MY trailers for years........

I install another drain valve on main drain header.
With it closed I open grey/black tank drain valves combining both.

Current rig.....have 3 waste tanks. Black, grey & galley
Can combine grey/galley, grey/black, grey/galley/black, black/galley or keep them separate.

I run out of fresh water before I can fill my 'waste tanks'

Everything is waste water be it black, grey (body scum/pee-pee) or galley (food/oils etc.)
Dutch_12078
Explorer III
May 11, 2015
AllegroD wrote:
For those with a straight pipe in the shower/tub, if matter backs up into the grey, this could cause other problems like smell in the bathroom. I'd want to be sure I had a trap in the shower/tub and a rinse in the gray tank before combining.

If there's an RV out there with a straight pipe from the shower into the grey tank, they've already gotten used to the awful smell. Grey tanks can smell as bad as, or worse than the black tank, as Effy said. I've never seen a factory RV without traps on the shower and sinks.
Effy
Explorer II
May 11, 2015
AllegroD wrote:
For those with a straight pipe in the shower/tub, if matter backs up into the grey, this could cause other problems like smell in the bathroom. I'd want to be sure I had a trap in the shower/tub and a rinse in the gray tank before combining.

My shower has a trap but I can tell you, gray can often be as bad if not worse smelling than black. No smell ever from my shower. I would think modern RV's would have to build traps into any gray water drain. Not sure I have ever seen a straight pipe into a gray tank. Maybe older models? yuk.
JimM68
Explorer
May 11, 2015
ours actually tends to fill the black first....

But we have a factory macerator (which I love....)
I will generally:

Turn on the pump and open the black and pump it until it is dry and the pump screams.
Open the gray for 30 second and then close it. continue pumping the black until the pump screams.
Close the black open the gray and pump it dry.

With our system, the techma electric macerating toilet tends to use a lot of water, we often fill the black before the gray.

If they still make these toilets, I highly recommend them....
They chew up about anything.
Nothing comes out the black tank but liquid...
We use 2 ply paper, not a problem ever....

While I've had to rebuild my sanicon once, I've never (KNOCK ON WOOD) had a problem with the techma, even it's tank level display has always worked perfect (KNOCK ON WOOD, LOTSA KNOCK ON WOOD, I SHOULD
N'T EVEN TALK OF THESE THINGS...)
AllegroD
Nomad
May 11, 2015
For those with a straight pipe in the shower/tub, if matter backs up into the grey, this could cause other problems like smell in the bathroom. I'd want to be sure I had a trap in the shower/tub and a rinse in the gray tank before combining.
Ivylog
Explorer III
May 11, 2015
I manually open both valves and when I hear the gray stop running I shut them. I use one of these caps with a hose valve on the small outlet that lets me drain the mostly gray from between both valves (less than a gallon) into the sewer hose before hooking it up .
Effy
Explorer II
May 11, 2015
2gypsies wrote:
Wouldn't you be contaminating your grey tank with poop? If you're a person who dumps grey water on the ground at times then you'd also be dumping fecal matter. I wouldn't want to park in the spot after you leave. What if your grey tank would back up into your shower? Not a good practice, in my opinion.

If you have so much more room in the black tank, dump your dishpan in the toilet or put a bucket in the shower and capture your shower water to dump in the toilet.

I have never, nor plan to, dump my tanks on the ground. Actually I've never heard of this practice. As far as it backing up, I keep a pretty good eye on my tank levels and would obviously dump before that happened.
2gypsies1
Explorer III
May 11, 2015
Wouldn't you be contaminating your grey tank with poop? If you're a person who dumps grey water on the ground at times then you'd also be dumping fecal matter. I wouldn't want to park in the spot after you leave. What if your grey tank would back up into your shower? Not a good practice, in my opinion.

If you have so much more room in the black tank, dump your dishpan in the toilet or put a bucket in the shower and capture your shower water to dump in the toilet.
Mr_Mark1
Explorer
May 11, 2015
Effy, air controlled or electric, it's a wonderful set up once you are connected to your 'sewer' outlet.

If it's raining out, no problem, if it's scorching hot, no problem, if it's windy and cold, no problem. Hit that button and you're done! :)

MM.
Effy
Explorer II
May 11, 2015
Thanks guys. Not sure why I never considered this before. Makes perfect sense. Might not do it all the time but in a pinch, dry camping seems logical to extend the stay. And with my set up, it's really easy and nothing to modify.

BTW, I installed an electric remote switch on my macerator. Now I can activate and dump electronically from anywhere within 50' without touching anything other than the keyfob. It's not as high end as your air controlled setup on your Prevost Mark, but it's pretty cool. Remote control tank dump. Now that's rouging it.