can you discuss it with her? Explain about bacteria in your mouth and on your body and how it grows and causes damage and yucky odors.
Telling her she "needs to", "ought to" "do it or else", etc makes it your idea, not hers.
( invitation to oppose or refuse ).
choose these tools or those, either this, or that items to use gives her the power to decide for herself.
Give her as wide a choice as you can offer. Same for bathing and cleaning. She can use something like Cetaphil where she never has to use water, wipe on, wipe off... many pre packaged wipes available. Hair washing is something else. Hair can be cut very short so head and face can all be washed at once. Again, give her the choice, let her choose so it is her idea, not pressure from you. Explore alternatives. There are places in the world where people have never had a bath or a shower but who manage to clean themselves in other ways. There are lots of alternatives out there.
Have you tried waterpik, plain toothpicks or floss piks and mouthwash? Give her a choice between tools, rather than telling her to try one or use one... so its her idea and not yours.
There are little brushes you can put on your finger (like the end of a glove). Tooth paste is not a "have to" item either. For years we used baking soda. There are so many options available.