Forward Black Water Tank


Having several issues with the forward black water tank on my 2017 3611 HD.

The vent doesn't appear to be functioning as the toilet burps every time I flush.
I removed the vent cap and pushed a 13' PVC pipe down the vent and didn't find any blockage.
Now, the tank won't drain even with the toilet flush valve open to act as the vent.

I installed a Valterra Flush King™ 45 Degree Reverse Flush Valve and ran water backwards up the drain line in an attempt to break loose any blockage. With the toilet flush valve open, the water level in the tank didn't rise at all.
Surprisingly, when I opened the Valterra flush valve, over 30 gallons of clean water came out. I first verified that the gray water valve was closed and it was. Then I repeated the exercise with the same result. Where could all of that water have been holding up?

I added a 32 oz bottle of Unique Clear-It RV Black Tank Unclogger Liquid 4 days ago but still have a full black water tank.

Anyone have a suggestion that could help?


Well-known member
Maybe you have a black tank slug floating to the gate valve?


  • Black tank Slug.JPG
    Black tank Slug.JPG
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Well-known member
When they drill the black for the down pipe the slug may have got left in the tank to float about. This happened to me once.