Combining black & grey tanks

There’s nothing different about a most Motorhomes. If you only have one outlet then it’s easy. Add a valtera twist on manual valve at the place you hook up the hose too and open both the black and grey valves. The tanks will both equalize levels. Open the manual valve when you want to drain. You just no longer use the old valves. I would cut and plumb ( glue) a hard valve on if you want to do it permanently as sooner or later your going to twist that valve off by accident .. oops. I close the old valves when working on hooking up etc “just in case” to limit the amount of spill is it accidentally twists off.
I put a twist valve on when boondocking to increase the time I can go between dumping. I have 2 - 75 gallon tanks and the grey always fills faster.

In a former MH that had a built in macerator I opened both valves and let the grey and blank equalize. I always found that we filled our grey 3x as fast as our black and this solved the problem. A few things allowed me to do this without relying on the single valve at the outlet. One was the built in Macerator. It acted as a valve and would not allow anything to pass unless I turned it on. It also had a bypass that had a valve and I never removed the 2 original valves and could close them at will. I also installed a tank flush on both tanks.

I think most high end MH's like Prevosts and Newell only have one very large tank. I could be wrong but that was my understanding.

I am not sure I ever understood why RV's have separate tanks unless it's just easier to install them that way. Seems like it would be easier just to have one large tank.

I let my gray tank get close to full. Then open the gray valve and allow the section of pipe between the closed Valtera valve and gray valve fill. Then I open the black valve, wait about 5 seconds and close the black valve. Just wanting to keep what's in the black tank in the black tank.

It really doesn't matter but I'm just funny that way. I don't want poop to back flow into my shower if I wait too long to transfer or dump.

2112 wrote:
I let my gray tank get close to full. Then open the gray valve and allow the section of pipe between the closed Valtera valve and gray valve fill. Then I open the black valve, wait about 5 seconds and close the black valve. Just wanting to keep what's in the black tank in the black tank.

It really doesn't matter but I'm just funny that way. I don't want poop to back flow into my shower if I wait too long to transfer or dump.

You make a good point. Probably the main reason RV's have separate tanks is to mitigate the black tank backing up into the shower.

The tanks on my motorhome are nested - The fresh water is on bottom, the grey sits on top of that, and the black sits on top of the grey.

With this arrangement, "combining" the tanks wouldn't work too well.

The only thing in the black tank NOW is urine, but Since eventually I’ll need to go to a dump station to empty THE TANKS, if I could utilize both tanks equally, instead of having to add shower or dish water down the urine diverted of the composting toilet, it would save me some trouble.

Someone mentioned that their class A tanks were nested, is there anyway to combine the tanks then AND how do I know if mine are, other than crawling around on the ground to try and figure if they are ?

Why? To avoid pulling two handles? I want pee flushed out of my hose with grey water. It’s usually somewhat soapy.

NOT so much having to “pull” two handles, BUT to NOT have the grey tank filled up so fast that I have to dump more often; I can add dish water & “run off” shower water, but that also runs the risk of the water sloshing into the composting/ solid waste side.

pymmint wrote:
NOT so much having to “pull” two handles, BUT to NOT have the grey tank filled up so fast that I have to dump more often; I can add dish water & “run off” shower water, but that also runs the risk of the water sloshing into the composting/ solid waste side.

You have me confused. What I read is that you now have three chambers. The composting toilet has one. The composting urine bypass runs to the second/black tank and grey water runs into the third/grey tank. Does the urine bypass have a check valve to prevent back flow from the black tank?

Does your MH have a combined exit valve like this?


If so, others above have explained simple method of adding the Valterra (one brand) gate valve (like below) to combine and control the dump.