That location and description sounds like the fresh water tank drain to me...
When you put fresh water in the holding tank via the door fill on the side of the coach (probably above that area), it goes into a tank under the bed. You then can use that valve to drain the remaining if you need to.
To operate the valve, it twists. There's an inner part inside the main casing. When looking straight down at the top of the valve, looking toward the ground, I believe it will turn counterclockwise to open. The extreme open and closed are basically where the handle is closest to the side of the trailer. Never leave the handle sticking out (like halfway on), because the handle/valve could get broken off.
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2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR