Welcome to the forum.
That doesn't look like a OEM cap! It is a new trailer?
There should be gate valves that close the gray and black tanks 'upstream' of the exit; one gate valve for the gray tank and different gate valve for the black tank.
Most gate valves slide open and closed with a 't' handle like you show in the photo, not with a switch.
Hope that helps.
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Sherm & Terry w/rescue Eydie (min Schnauzer) & Charley (std Poodle)
SOLD:2015 Jay Flight 27RLS, GY Endurance (E), Days: 102 '15, 90 '16, 80 '17, 161 '18, 365+ '20
SOLD: 2006 Ford F350 PSD, 4WD, CC, LB, SRW, Camper pkg., 375,000mi
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