I've not worked in that area - but the things i can think of (which I'm sure are things you've already thought of yourself) would be something about discrimation and challenges faced by homeless people and also your ability to combat this - maybe with some examples from personal or professional life.
Something about drugs/alcohol use and misuse and working with these issues - as well as mental illness. Maybe some kind of scenario will be presented to you and you'd be asked how you would work effectively with a particular type of person i.e. with an alcohol dependency.
What difference will you, personally, be able to make in this role is another question that might be thrown up.
Anyway, sorry it's not more specific but just some ideas!