It looks like the original installation.
Remove the curved peice of plywood and you will see the drain.
Its very inconvenient.
Mine is set up with a permanent bed in the rear so I put an elbow on the drain fiting and have the valve under the trailer now.
If your sitting as low as I did you may scrape it off.
I gutted mine because the fabric wall material was hanging down all over.
The inside is now finished with Scamp
It's all back together so I'll be happy to answer any questions I can.