Hello, welcome to Pet Talk. Yes, I tend to think, follow the cat, then lead the cat. If it were me, I would put her litter tray in the bathtub for a few days, to get her thinking you're "with her" in her thinking so she'll start to "trust your judgement" when in a few days' time you start to move it, first outside but right up against the bathtub, then six inches away, then another six inches and so on every four or five days till it's outside the bathroom. Unless you feel the bathroom is a good place to keep it, if there's room? See how much she'll tolerate and perhaps let her show you where it's comfortable for her to have her litter tray.

Another thing, where was her tray before this started? Did a noise, or a biting insect, or anything at all spook her one day when she was using her tray in its former location? A new household cleanser with a foreign smell that she found irritating to her nose each time she went to use the tray?

I can't think what else to suggest, fortunately I have never had this problem. It must be very distressing, but at least she has settled on one place now, and in the bathtub its easily contained and cleaned up. Good luck.