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How To Clear A Blocked Line In Your RV Black Tank

August 06, 2020

How to clear a blocked line in your RV black tank, Troubleshoot your RV holding tanks. Unique Camping + Marine

Many RV owners, especially those who are new to the lifestyle, will run into some sort of a holding tank issue, whether it be a pyramid plug, a compacted tank, or a blocked line. Of all of these types of clogs, however, a blocked line can be the most deceptive. 

A blocked RV sewer line is relatively uncommon, but it can be difficult to diagnose as it is often mistaken for a pyramid plug. But the good news is that a blocked RV sewer line is normally much easier to fix than a pyramid plug, and it’s much less serious!

A blocked line occurs when solid waste (normally toilet paper) starts to build up in the pipe between your RV toilet and your black water holding tank. This can eventually clog the entire waste pipe, causing a backup all the way to the toilet! Blocked lines normally occur in RVs where the toilet isn’t directly over the holding tank. We often see this in 5th Wheels. It is also common in RVs that have complex plumbing, like bends and turns in the pipes (a line with a 90 degree elbow, for example) between the toilet and the holding tank. Toilet paper can tend to build up at these elbows and cause issues. As we’ll cover below, a blocked RV sewer line is pretty easy to prevent by using proper RV holding tank maintenance.

In this article, we’ll dive into everything there is to know about blocked RV waste pipes plus how to fix and prevent them!

How to Diagnose Your Tank Clog

Like we mentioned, a clogged RV sewer line is often confused with a pyramid plug or the other way around. But there are some concrete steps to take that will help you to know if you’re dealing with a blocked line as opposed to a pyramid plug.

  • Step #1: Open your black tank valve. If you were using your tank for some time before it started backing up, waste that accumulated before the line got clogged should flow out when you dump the tank. If waste comes out of the holding tank, but you still have standing water in your toilet, you probably have a clogged line - and it is clogged between the toilet and the holding tank. If nothing comes out of your tank you open the black valve, it’s likely you have a pyramid plug. Be sure to close your black tank valve before you move on!
  • Step #2: Add water to your RV toilet if possible. If you have a blocked line or a pyramid plug, you won’t be able to add much if any water to your tank through your toilet.
  • Step #3: Ask yourself whether or not you kept your black tank valve open. A pyramid plug forms when you keep your black tank valve open, allowing all the water to flow out of the tank and causing a pyramid of poop to form. If you kept your valve open, you likely have a pyramid plug, but if you kept it closed (as The Unique Method recommends!), then you most likely are dealing with a blocked line.

After you've gone through this diagnoses, does it seem like this ISN'T your problem? Read here to diagnose if you have one of the other common tank clogs and learn how to solve it!

How to Unclog an RV Sewer Line

If you believe you have a blocked line instead of a pyramid plug, then you’re in luck: A blocked line is easier to remove than a pyramid plug—although probably no less gross! Follow the steps below to remove your clog.

  • Step #1: Try to force the blockage out by plunging your toilet. We put this recommendation first because there is no reason why a plunger can’t be used to push a blockage out of your line and into your holding tank, and many people have success with this method. With that being said, a plunger may not work, especially if you have a more serious blockage in your line. 
  • Step #2: If plunging doesn’t work, try pouring 1 bottle of Unique Tank Cleaner into your toilet bowl. Let Tank Cleaner work overnight and try to plunge the toilet again in the morning. If this doesn’t remove the blockage in your line, continue to step #3. 
  • Step #3 (optional): If you still can’t get your line to open, try using PEX pipe or a toilet snake to force the clog out of the line. Simply turn off your fresh water supply so you can hold down your toilet flush-pedal and use a length of PEX pipe (which is a flexible type of pipe that will allow you to get around any elbows or twists in your sewer line) or a toilet snake to chip away at the clog. By using this method (especially in conjunction with the previous points), you should be able to remove the blockage in your RV waste pipe.

How to Prevent a Clogged RV Sewer Line

As we mentioned earlier, a clogged RV sewer line is relatively uncommon, but it still happens occasionally, and the best way to prevent it is to follow the right care procedures as laid out in The Unique Method

The best way to prevent a blocked line is to use ample amounts of water in your toilet and tank! This is integral to The Unique Method, and it will help keep your line free from excess toilet paper and debris that could build up in the elbows of your RV waste plumbing. 

As The Unique Method states: 

  • Always fill your toilet bowl with water before using the toilet
  • Hold your flush pedal down for 10 full seconds each time you flush your toilet.

Another way to make sure that you don’t get a blocked line is to keep your toilet paper use to a minimum. Yes, use as much as you need, but try to stay away from excessive amounts of toilet paper flushed down the toilet all at once! This can cause an unwanted clog!


We hope that this guide has helped you to understand how to unclog an RV sewer line! Blocked lines are quite uncommon, but still a frustration! Thankfully, they’re pretty easy to prevent if you follow the right care procedures—as The Unique Method lays out!

As you know, after reading this guide, there are other types of tank clogs besides a blocked line: a pyramid plug and a compacted tank. We mentioned above how a pyramid plug is often confused with a blocked line, but we never covered a compacted tank. To learn more about these clogs and how to fix them, check out our very thorough article, Unclogging An RV Toilet!

If you have any questions that aren’t answered in this guide, please feel free to reach out to us at support@uniquemm.com! We’d love to help you solve your issue and get back on the road clog-and-odor free!

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