Mike makes a good point - in that it can be hard to help people see the value that PMM brings to the table. When I'm looking to take on a job in a new company, it's one of the first things I try to sniff out is what the company's view is on Product Marketing (e.g. do they just view product marketing as glorified content writers - which by the way, we're not... we're messaging, market, and buyer experts - not copywriters).
In terms of showing ROI beyond that, treat Product Marketing more like a data science than an art. So, for example, what is PMM job? it's to essentially accelerate sales and help drive revenue. Really, that's a lot of our end goal for the business. So, you should know your numbers through and through. How much revenue is the product bringing in every quarter? What's the growth rate? how long is the sales cycle? How are different ads/content/assets performing that use your messaging?
Now that you have all of those numbers (and more, that's not an exhaustive list), then it's time to put into action a plan that accelerates those numbers. How do you turn that growth from 10% to 20%. Why is the sales cycle for X product Y long, but the cycle for A product is B long? How do you shorten that window? Why do some customers hate your product, and others love your product (a clear sign of incorrect messaging/positioning/targeting)?
Then once you make changes, keep tracking the numbers - does the new ad copy with the revised messaging bring in more clicks/trials/customers? then there is your ROI. Rinse and repeat for everything else. Did the new audience your targeting result in shorter sales cycles? great, double down, there's more ROI.
Luckily for us, a lot of our work can be directly attributed to a return on the investment, but it's really up to you to make sure that you're the one tracking your numbers and the work that goes into it.