It’s simple. Make a list of the questions your customers ask you during sales appointments. These are the topics they are most interested in. These are the topics they are most likely to do a Google search upon. And these are the topics that they will click through in your email newsletters.
At most companies there are a handful of rock star product managers (or product marketing managers) that visit regularly with customers. Start by identifying these individuals. Tell them that each time they visit a customer they should come back with a list of 3-5 questions that were asked. You can enforce this by instituting a policy that the CMO will not approve travel expense report until the questions are submitted.
If you are not getting a broad enough sampling of questions then consider starting a contest with the sales engineering or solution consulting team. Sales engineers can earn points by submitting questions from RFPs or live customer meetings. Points accumulate to earn gift cards or other awards. Give them extra points if they not only submit the question, but also the answers.