Battery draining overnight

You said 'fuse' for the dome light. As mentioned, make sure the doors are closed, BUT, there's a helluva lot more on that circuit than just the dome light.

Set up your test light between ground and battery negative, but unplug the three connectors at the bulkhead. If the light goes out, plug in each one, one at a time. If light goes on, you know it's in one of those three, and they're grouped, so it will help you isolate the issue. I like this better than the fusebox, because it's 'narrower' in focus. THEN you can use the wiring diagram to see what's affected.

Like Dana said above, if each fuse causes the light on, disconnect the ignition switch, and see what happens.

ETA Dome lamp goes through the instrument panel harness.