Black water tank flush

We just bought a Newport landmark 365 2021. We can't figure out what is going on with the black tank flush, maybe someone could steer us. There are two ports as we have two bathrooms. When we put the hose on to one the water comes right out the other. Anyone know what might be up with that or what we need to do? It seems like it should be so simple. Also, anyone with two bathrooms, is tank #1 upstairs on downstairs. There is no manual that gives us anyone sort of information.
thank you!

Not sure why they are interconnected, but it sounds like they’re missing one or both check valves at the fitting. The OEM parts are cheap plastic junk.


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I assume it's still under warranty? Let the dealer figure it out.

Sounds like the black tank flush was plumbed wrong. If chasing this is not your thing, I'd let the selling dealer sort it out.

I have seen this before on another Landmark.

Your anti siphon valves are plumbed wrong.

They are behind the wall of the shower.

This is how this guys tanks were plumbed


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Sorry for the horrible images but I'm doing it on my phone.

This is how it should be

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Jesstruckn/Jesstalkn said:

Sorry for the horrible images but I'm doing it on my phone.

This is how it should be

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Click to expand...

I have found one anti siphon behind the shower. Where may the other be located?? Thank you

The second one should be somewhere near and above the tank for it. If you can access the tank, you should be able to see the water lines for it. They don’t always put them in an accessible location. In a blind wall cavity behind the toilet is one area they like to hide them. DAMHIKT


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Sensors for RV black water tanks are normally found at the bottom of the container. These components usually feature positive and negative sections, indicating when the reservoir is empty or full to the rig users. When it senses that the wastewater in the container has reached a certain level, the system completes a circuit. The filth and sludge in the black water tank, on the other hand, may cause the sensors to fail, necessitating cleaning to restore their effectiveness.

Word of caution if you’re not aware: waste tank sensors are spin-welded to the plastic tanks. You cannot remove them to clean them. Most internal cleaning methods are ineffective at best.


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I have a black flush connection on my Bighorn which has been less than effective in flushing / cleaning the tank. I just purchased a Valletta Flush King which hooks into your outlet pipe and has the water flusher. It is the best accessory I have purchased for the 5 th wheel. I used it immediately and could not believe the stuff that came out of the tank,, when I checked my monitor the Holding Tank read Green ,, first time since 2012. I will not be using the built in flusher any longer.

I have a 2019 Newport. My tank dump valves and flush ports were both labeled at the factory. Like you said, nothing in the manuals on any of that stuff. Nice to have it all labeled. Wrong. Every label must have been installed by Ray Charles. None were correct. That took a bit of listening and flushing, to get corrected labels on. Then I figured out how to get behind the baggage wall, on the valve box side. Everyone should check that out for their selves. Quality control sure doesn't. Real easy to check all the connections on the backside. My cable connections for the living room were disconnected under there too. Agree completely on the Flush King. But I use quick connects, and still use the flush ports too.