My Outlaw 29H had the black tank drain pipe plumbed under the drive shaft and over to the "T" with the gray tank. There was a drain valve at the black tank and another one at the "T" (in addition to the gray tank valve which could drain on its own through the "T" connection).
I believe they plumbed it this way because the drain pipe did not have much ground clearance and should it get damaged or torn off, you would have a heck of a mess.
I ultimately removed the drain pipe from under the drive shaft and added a sewer hose connection on the passenger side. I then got a "Y" adapter for the hose and dumped from each side.
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