Mine has a valve on the passengers side, but not a drain. There is the usual valve and drain on the drivers side.
No one has ever explained why there is the valve on the passengers side, near the black tank, but my guess is to help keep the pipe to the drain free of you know what so it doesn't get blocked.
I normally keep both valves closed. When I dump I open the valve at the drain first then the valve at the tank. This gives the fluid coming out some pressure and hopefully clears the pipe. When finished I close the valve at the tank first, let the pipe drain, then close the valve at the drain. This has worked for me for many years.
Hope this helps.
PS. It's probably also a safety thing. This plastic pipe running across the body behind or near the rear wheels may be subject to damage by flying rocks ???
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