Persistence? :) That's a tough one and something that I've experienced many times in my career. I've always approached that as a challenge that I strongly desire to overcome. Similar to a question I answered above, I always start by building a great working relationship with Product by clearly showing the expertise and value that I uniquely bring into the partnership. As a PMM expert, you want to be seen as the central hub for a lot of critical information regarding not only your customers, but the products you sell, competitors, industry analysts, and many other constituencies. To be considered a subject matter expert, you need to continually gather and analyze data and business intelligence from all of the sources mentioned, Sales and CS, plus external sources — and use this data to inform the evolution of your recommendations around strategy and messaging. This is what separates great PMMs from good PMMs, and the type of expertise that Product values. Competitive intelligence and industry expertise tend to be weakness areas for Product that PMM can uniquely deliver, so that would be a good place to start to beef up your knowledge base to show how you provide value.
I would first try to understand where the product manager is coming from, i.e. what knowledge or perceptions are they starting with about Marketing? What context might they need? What pain points might they have experienced in the past with other marketers? Starting from a place of empathy can help you determine what information would be most helpful to share with them to bring them along as a partner and gain their support.
Once you have that baseline, I'd then craft a deck or document that provides an overview of Marketing or Product Marketing. You can include things like team structure, mission, focus areas/responsibilities, and examples of what marketing activities with product look like in practice.
Once you've taken the steps to align, I'd focus on carving out small win projects that you can do with Product to show value through doing vs. telling. Proving your worth through consistent action and results is often times the easiest way to change someone's mind -- but just remember it may take some time and you have to be patient.
Silicon Valley is littered with failed products that were technically excellent but totally failed to win their markets.
Time and again technical founders and product people find that they've run out of market runway, gone head first into a strong incumbent and have not understood buyer's mind or the "forest from techincal trees".
You have to present to them the state of the market so that product strategy decisions can be made in cognizance of the market. This is not a one time presentation but a collaborative conversation that happens over time.
Strategic PMs will get it, make them your allies.
First, make a concious decision to give them respect without demanding any in return. You don't really "deal" with them. Instead, you learn to foster a healthy and productive working relationship.
Now, treat PMs like a customer who you have to understand so you can win over. Enter into their world. Follow them on social media and read everything they repost. Ask them what books or podcasts they would recommend. Follow through by actually reading and then reporting back to them what you thought about the content. Learn to speak their language. When we feel understood, we feel loved.
Finally, be aware that we all bring baggage into situations. Sometimes a bad experience with marketing at a previous company can cause PMs to look at you through a tainted lense. If you can rebuild healthy rapport with them on a personal level, then you have opened the door to share with them how marketing provides value.