Any Proposal, Grant, or Technical writers with interview insight?

I've successfully made it to the final interview of a prestigious nonprofit organization. They are seeking a business proposal writer, and requested that I send in 3 writing samples (one RFP) for their review. I sent them and was offered the final interview opportunity. I am very nervous because the interview will be 90 minutes with multiple senior leaders in the organization present. I have a degree in professional/technical writing. And I have a minor in communication. My background is in social media management, operations management, and children's librarianship. I am concerned about what will be asked of me, and whether I am prepared enough for the interview. I am currently scouring the internet for any advice or tips. I can leverage the fact that this is their first business proposal writer position, and no one has filled this role before. So it is still being fleshed out. Any advice about what to prepare for would be greatly appreciated.