Rubbing alcohol mixed with water
Vinegar and water. If tough, soak cleaning cloth (or paper towels) in vinegar/water and leave on the tub for a while
Here are some posts on Hometalk for tub cleaning that will work great: https://www.hometalk.com/search/posts?filter=clean%20tub
Hello- this Hometalk, Bathroom bathtub-How to Clean the Tub Without Breaking Your Back DIY link might be helpful.
I like to use bar keepers friend. https://amzn.to/2IoZEBJ