To be clear, Product marketing doesn't priortize feature on a roadmap, you enable Product with the data and business case to prioritize features. Arm the teams with the data and insights needed to drive your case. Can you pull in product usage data, sales pipeline and revenue impacts, support tickets and NPS data, user testing feedback, etc. If you have these elements, then you can provide a rationale for why certain features are ranked in the order they are to drive the discussion with Product.
To establish credibility with a new team, the first step is understanding the team's need, laying out a vision for how you can best add value, and aligning around expectations. It is important to know the user, the market, and the product so that you can engage with the cross-functional team in a meaningful way from day one. With a clear set of objectives and foundational understanding of the space, you can quickly begin to make an impact on the team.