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Old 08-01-2019, 07:29 PM   #1
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Perplexing Grey tank problem

So...I went out in my 5th wheel this week. I started out with two empty grey tanks. I had a full hookup site and opened both grey tanks as soon as I hooked up.

The night before we broke camp, my DW told me that water was backing up the kitchen sink drains and the tank monitor was reading 2/3 full?!?!?!

I checked the outside tank levers and confirmed that they were in the open position. I closed both valves to accumulate enough water to clean out my sewer line after draining the black tanks the next morning. One grey tank (bathroom shower) had a lot of water in it; however the kitchen grey tank had little water in it, despite reading 2/3 full.

At the end of tank emptying, the kitchen grey tank went from 2/3 to a 1/3 reading????


1. How could my kitchen sinks back up and read 2/3 full, when the tank valve was open the whole time?

2. Why is it still reading 1/3 full after emptying?

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Old 08-01-2019, 08:31 PM   #2
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Clean your sensors first, add some dish soap in each tank with them about 1/3 or so full before you drive to your camp site.


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Old 08-01-2019, 08:35 PM   #3
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Hmmm??? Please let us know the model number of your 5th wheel. Someone may have helpful information.
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Old 08-01-2019, 09:19 PM   #4
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Bottom line, the tank sensors suck! When I empty my grey tanks, the sensors still show 1/3 full. Don't know why the industry has not come up with a more reliable system to detect the capacity. Some sort of "dip stick" would be a lot more reliable!
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Old 08-02-2019, 05:58 AM   #5
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Have you had work done on your tanks? Others here have reported finding blockages from items left in tanks such as waste plugs from hole saws after having fittings installed.

Someone else in the family fell in with a bunch of facebook RV conspiracy theorists and as such, not even coffee grounds go down the sink. I go along with it but we use a lot of paper towels.

We've learned to largely ignore sensors as well. We open the greys when doing laundry just to be safe. But if you had a back-up when you know the valves were open, that sounds like a physical blockage.

Once in my city office condo we had an unexplained back up in storm drain. A plumbers optical scope and camera found a wood 2x4 scrap that had clearly been in the pipe for years but one day through a trick of bad luck created a flapper valve in an elbow. Weird things happen.

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Old 08-02-2019, 06:32 AM   #6
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As said above, the sensors are junk. You can not rely on them for accurate readings whatsoever. Mine haven’t worked correctly from about the second trip out 3 years ago. You can either upgrade to a better sensor which sometimes is labor intensive or learn to live with it like the other 80% of us. I know my grey tank is almost full when my sinks start draining slow and gurgling or my shower stops draining. You’ll get a “feel” for it with more experience.
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Old 08-03-2019, 05:53 AM   #7
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The real issue is that both tank valves were open yet "water was backing up the kitchen sink drains". It almost sounds like one gray tank was draining into the other (not sure if this is possible). I personally never leave my valves open. You never know what might make its way into your empty tanks. Anyway, as long as they do empty as expected, one at a time, I wouldn't worry about it.


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Old 08-03-2019, 06:49 AM   #8
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So, both were open, and not draining(not normal). However the confusing part is where you stated you went to break camp and closed both to accumulate water to clean the hose post black dump. Was one tank already empty at that point? Did cycling the valves move something that then allowed the tanks to drain. Confirmed by a clear elbow? On my unit, there’s a third valve where the hose connects. If I open both grey tanks and forget that, it won’t drain. Is that a possibility? Don’t mean to insult anyone’s intelligence. Just thinking of any and all possibilities. Also, when dumping the pressure would work it’s way through any obstacles you created laying the sewer hose out incorrectly. It is possible the hose had an odd bend or too much uphill to be overcome by the moderate pressure of just running a sink or a quick shower.

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