We are getting a massive amount of sewer flies. I have tried putting cleaners in both tanks and flushing them I tried the tank flush hookup today and water just poured out if the bottom of the fifth wheel. Any idea what this could be?
Hi Troy - Two things come to mind that could be the source of this leak.
Since you mentioned you had hooked up the black tank flush when this occurred, it could be possible the connection of this hookup at the tank itself could be broken or leaking, causing the water to pour into the underbelly and leak through the point the drain pipe exits the coroplast.
Another possibility could be the connection of the drain pipe to the black tank has a leak or the valve connection is leaking.
The water in those pics looks clear so I’m leaning towards the black tank flush connection being the culprit as the water probably wouldn’t be clear if it was leaking directly from the tank or drain pipe. 
Depending on your model, you may be able to get eyes on the path of the black tank flush connection to the tank itself from the front storage compartment. Otherwise it’ll require dropping the coroplast underbelly to get a view of the tank and connections to pinpoint the source of the leak.
If you have an endoscope you could use that to get a look at the tank connections from the storage compartment cutouts where the tank valves are located. I have one of these Dual Lens Wireless Endoscope and it’s been extremely helpful in getting a view into hidden areas of the RV.
Good luck tracking this down and let us know what you discover.
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We had the same problem. First trip out, and we woke up the next morning to find a pool of black (brown) water on the ground.
I had to take off the under belly panel to access the area around the black tank, and it was a mess.
I found that, during construction, they drilled a screw through the frame of the trailer right into the 3" drain pipe for the black tank.
I won’t even get into the stupid design of the black tank and plumbing on our model (41FLB) but you would think that the guy with the screw gun would check to see what was behind the frame before he/she started drilling.
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And also our tank flushing water line does not go through to the black tank. When you hook up a hose, water does not go through to the tank. I changed the vacuum breaker valve so the problem is probably where it connects to the black tank. Clearly Vanleigh had issues when assembling their plumbing systems. So I agree with Stuart, your water line could be the issue.
Wondering if you were able to get to the bottom of this?
Excellent troubleshooting steps Stuart.