I'm sorry you don't feel understood. I know you want to be able to help the girl, but it's obvious you have talked to the parents and without their support, there is nothing you can do.
Yes you are being harassed, no it's not fair. As much as I am stuck in a pity party rut myself right now, I'm trying to accept that we all have things in our lives that arn't fair. The only thing we can do is "accept the things we cannot change, have the courage to change the things we can, and the wisdom to know the difference." If you can't change your situation (you can't affect her behavior, and you can't move) you need to find a way to accept it. I'd suggest earplugs so you can't hear her following you, and if you already have the tv on mute it wouldn't make a difference, and try to go about your day as you would if she didn't live up there.
And although I know it's probably not right I would be very tempted to go on the computer everytime the couple is 'enjoying' their bedroom time, just because then they will be forced to react to their daughter's desire to follow you. Yes, this may just mean leaving her locked out of the room and having her throw a tantrum, but then the parents would also be suffering some of the consequences their daughter is forcing on you.