RV Grey Water Tank Clogged? 7 Proven Solutions to Clear Blockages Fast

What's Going On

Your 2007 Dolphin's intermittent grey water drainage issue is commonly caused by a manufacturing debris plug floating around inside your tank, just as you suspected. This is a surprisingly common problem that affects many RV brands, not just National RV units like your Dolphin. When manufacturers drill the drain outlet holes in grey water tanks, sometimes the plastic plug created by the drilling process falls into the tank instead of being extracted with the drill bit. This creates a floating obstruction that randomly blocks your drain outlet, explaining why your tank drains normally sometimes but barely trickles other times, regardless of whether you're level or not.

Your reverse flush valve solution is actually the most practical fix for this exact problem. Since the plug is hard plastic material (same as the tank), chemical treatments like Unique RV Digest won't dissolve it, and traditional snaking is nearly impossible due to the tight bends in most RV drain systems. The pressurized water approach you're using effectively pushes the floating plug away from the drain opening temporarily, allowing the tank to empty.

Why This Happens

During RV manufacturing, grey water tanks have multiple fittings installed including drain outlets, level sensors, and vent connections. When workers drill these holes using hole saws or large drill bits, the circular plastic plug should theoretically come out attached to the drill bit. However, manufacturing processes aren't perfect, and sometimes these plugs fall into the tank instead. Since grey water tanks are typically installed in tight spaces and sealed up before final assembly, there's often no practical way to retrieve these fallen plugs during manufacturing.

The plug becomes a floating obstruction that moves around based on water flow, tank angle, and road vibrations. When it happens to settle over the drain outlet, you get the blockage symptoms you're experiencing. This explains the random nature of the problem - sometimes the plug is positioned away from the drain and everything works fine, other times it creates a near-complete blockage.

Traditional troubleshooting methods fail because this isn't a typical clog. Vacuum suction can't grab a smooth plastic disc, chemical treatments can't dissolve manufactured tank material, and snaking is ineffective because the obstruction isn't stuck in the drain line itself but rather floating in the tank. This is why your reverse flush approach is so effective - it addresses the root cause by physically moving the obstruction.

Getting Ready

You'll need a reverse flush valve system, which typically costs between $30-60 depending on the brand and features. Popular options include the Valterra Master Flush or similar reverse flush adapters that connect to your city water supply. You'll also want to ensure you have adequate water pressure - verify the recommended pressure with your tank manufacturer, as excessive pressure can damage tank fittings or seals.

Safety Warning: Do not exceed maximum safe pressure limits as specified by your tank manufacturer. Excessive water pressure can cause tank damage, fitting failure, or flooding.

Before attempting this repair, make sure your grey water tank is as empty as possible through normal draining, even if it's just trickling out. This reduces the volume of water that needs to be displaced when you reverse flush. Have your regular dump station tools ready including gloves, as you may need to quickly transition between reverse flushing and normal draining. It's also helpful to have someone assist you if possible, as timing the valve operations can be tricky when working alone.

Consider marking your reverse flush valve clearly and storing it with your other RV maintenance tools, since you've identified this as an ongoing issue that will likely require periodic attention. Some RV owners in similar situations report needing to use this technique every few dumps, while others find the plug stays displaced for weeks at a time.

Walking Through the Fix

Connect your reverse flush valve to the grey water drain outlet while keeping the tank drain valve closed. Attach your garden hose to the reverse flush valve and connect the other end to a pressurized water source. Before opening any valves, ensure the tank vent is clear and functioning properly to prevent pressure buildup. Slowly open the water supply valve to begin sending pressurized water up into the tank through the drain line. You should hear and feel the water entering the tank, which will help push the floating plug away from the drain opening.

Allow the pressurized water to run for 30-60 seconds to ensure the obstruction has been displaced from the drain area. The water pressure will also help agitate any debris in the tank and push the plug to a different location. Once you've given the water time to work, carefully close the water supply valve and then open your grey tank drain valve. If the plug has been successfully moved, you should see a strong flow of water rather than the previous trickle.

If the tank still doesn't drain properly after the first attempt, repeat the process but try varying the water pressure and duration. Sometimes it takes a few cycles to get the plug positioned where it won't immediately drift back to block the drain. Once you achieve good drainage, allow the tank to empty completely before disconnecting your reverse flush setup.

Keep notes on what water pressure and timing work best for your specific situation, as you'll likely need to repeat this process periodically. Some RV owners find that using this technique every few dumps prevents the plug from settling back into a problematic position, making the process quicker and more predictable over time.

Beyond DIY Territory

If the reverse flush method stops working effectively or you're tired of dealing with this issue every time you dump, the permanent solution involves professional tank inspection and plug removal. This typically requires removing the tank or cutting an access port, which is definitely beyond DIY territory for most RV owners. RV service centers with tank expertise can sometimes use specialized inspection cameras to locate the plug and determine the best removal approach.

Some service centers might suggest tank replacement if access is particularly difficult, but this is expensive (typically $800-1500+ depending on tank size and labor access) and usually not necessary for what's essentially a minor manufacturing defect. A more moderate professional approach might involve installing an additional drain fitting or modifying the existing drain system to prevent future blockages.

If you notice the problem is getting worse or your reverse flush technique becomes less effective over time, it could indicate that the plug is deteriorating or that additional debris is accumulating in the tank. In these cases, professional inspection becomes more worthwhile to prevent potential tank damage or more serious drainage problems down the road.