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Old 07-28-2021, 03:14 PM   #1
Junior Member
Brand: Thor Motor Coach
State: New Hampshire
Posts: 1
THOR #24153
Thor Compass Grey Tanks

Hi we have a 2019 Thor Compass that has two grey tanks.
Recently we noticed our shower backing up and bathroom sinking caused water to come up out of the shower.
We have a black tank and two grey tanks. We have one main grey tank valve that we pull to empty that tank. However, there are also two smaller grey tank valves that go to kitchen sink and shower/bathroom sink.
It seems that they can both be isolated - not sure why you would have one closed and the other open.
Right now, our shower and bathroom sink are draining fine but we pushed IN both of those smaller grey tank valves. Does this mean they are open or shut? If pushed in means shut, why is the shower now draining fine?

I guess I just don't understand how the grey tank systems work.
Can anyone confirm or explain this to me?
Thank you so much in advance!

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Old 07-28-2021, 03:52 PM   #2
Senior Member
paulwadley's Avatar
Brand: Thor Motor Coach
Model: Miramar 37.1
State: California
Posts: 2,034
THOR #12698
The bathroom sink drains into the black tank. They are set up that way to make sure your black tank has enough water to push out any solids in the black tank. The galley sink feeds into the grey tank. If the valves are pushed in, they are closed. If the black tank is on the passenger side of the coach, there is a 2nd valve on the passenger side that is used to close off the black tank from flowing through the crossover pipe to the dump station on the driver side. That valve needs to remain closed until you are ready to dump the black tank. It is a safety feature to keep the black tank from leaking in the event you should break or crack that crossover black tank pipe while driving. The black tank must not leak onto the roadway. I usually set up my sewer hose at the dump station and open the black tank valve there. I then walk over to the passenger side and open the other black tank valve to release the black tank fluids out. I checked the specs for your coach, you have 1 grey water tank, 1 black water tank and 1 fresh water tank.

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Old 07-28-2021, 04:29 PM   #3
Senior Member
yukongrant's Avatar
Brand: Thor Motor Coach
Model: Compass 23TR
State: Yukon
Posts: 197
THOR #13409
The bathroom sink and the shower use the same GREY water tank. The sink does NOT drain into the black tank on a 2019 Compass like most others. (That's what we own.) I was really surprised when I poured a bucket of water into the bathroom sink and it backed up into the shower. It should go to the black tank to help with enough water. But they don't. (At least on our rig.)
If the valves are pushed in, they are closed for dumping purposes. It should not matter if they're in or not for the backup problem from the the sink to the shower. The water flows directly to the holding tanks with no valves in between.
The black tank location may vary by model - but on our 23TR, it's on the driver's side. No extra cross pipe.
When I dump, I connect the stinky slinky, pull the black tank and let it drain. When done, I then pull both grey tank valves and then the main grey tank valve and flush as much of the black tank waste out as possible. Then I turn on the sani flush and let that run until the waste runs clear. Some people shut the black tank valve and let the tank fill before dumping. I will be trying that at some point. Until now, I have not done that.
Hope that helps.
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