How do I clean a very stained bathtub?

What finish? Porcelain or fiberglass?

fill the tub with water with bleach added. Also there are powdered cleaning agents that might specifically address this problem.

A pumice stone can get off stubborn hard water stains.

Soft scrub with bleach.

If your tub is acrylic or fiberglass and the stain has not come out with Softscrub nor any other cleaners: it’s permeated the plastic.

Time to re-coat the whole thing with a Coating System- like Rustoleum’s.

If its really cruddy you can use Easy Off Fume-Free Oven Cleaner in the blue can and use a Magic Eraser to get just about anything off... https://www.youtube.com/watch?v=L10jCtHCcmY

I have disabled clients whose tubs are a mess and I use Dollar Tree Awesome Orange in the half gallon which I pour down the sides of the tub, let it sit and scrub off with a long-handled brush.

Wear gloves and for the EasyOff be sure to open window and turn on the fan