You need to have a sewer hose hooked up to your camper. Then you will need to locate a pull handle somewhere along the frame or inside your docking center. That valve needs to be open to drain your tank. You will have more than one pull handle and you will have to find the correct one. You might have two gray and one black tank pull handles.
If you have neighbours, it's not a crime to ask for assistance.
Attached is a picture of where my drain hose hooks up. Yours will be similar but without that handle.
That handle is similar to waste you will be looking for to drain.
Peace
Dave