There are a number of questions you could ask Miss Tubman. For starters, where was she born?
How did she manage to escape to freedom originally, like when she freed herself?
What particularly influences her, for example, the Bible, the abolitionist press, speeches by abolitionists, etc.
Has she ever met Frederick Douglass?
Has she ever met William Lloyd Garrision?
How does she feel about the Women's Rights movement, and ladies like Elizabeth Cady Stanton and Susan B. Anthony?
If she has one wish for African American People, what would it be? Is it freedom from slavery, the vote, equal rights with white people?
And lastly, if she is blessed enough to live to see all black people free, how would she like to spend the rest of her life?
I'd avoid specifics about the Underground Railroad, in her day being involved with it could get you hanged, and too many specific questions would arouse suspicion. She must have been very clever and "cagy" to avoid capture for so many years.