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Old 10-29-2018, 07:37 PM   #1
Junior Member
Join Date: Dec 2015
Posts: 11
Can you flush black tank with gray tank

I am doing some research and I see where people put dish soap and chemicals into their black tank. I was thinking I could---
1) Add a knife valve at the outlet of the combo drain.
2) Drain the black tank completely and close the new valve (black valve still open).
3) Open the gray valve and let some gray liquid go into the black tank.
4) Close the gray valve and then drain the black tank again.

Has anyone heard of dong this?
Could it cause any issues?

Thanks to all you folks for the free and excellent advice!

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Old 10-29-2018, 07:45 PM   #2
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Join Date: Sep 2010
Location: Prince Edward County, Ontario, Canada
Posts: 2,967
Pretty sure youíll never fill the black tank with enough gray water to do any good. Unless your gray tank is located above your black tank, then gravity would take all the gray and fill/clean the black.


Ken and Terry
2010 Georgetown 330TS being pushed by our
2017 GMC Terrain toad (4 wheels down)
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Old 10-30-2018, 10:38 AM   #3
Head Rambler
Join Date: Mar 2008
Location: Southwest Alabama
Posts: 6,465
I actually do that if I'm at a dump station that doesn't have a water supply, and I don't want to spend time running the onboard pump to add water to the tank.

As said above, it doesn't fill the tank, but it helps clear some of the lingering stuff out.

Then I'll usually give it a good flush once I get to the next campground or get back home.
Salem 29RKSS Pushing a GMC Sierra 2500HD!
Gotta go campin!
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black, black tank, flush, tank

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