The Release Manager is the Product Owner

What's a release manager? A marketer. A public relations fellow. Someone that cares both about the customer AND the product development team but in no specific order. This person is the best possible person you could have on your team or company. They can turn a bad project into a good one and a good one into a bad one. If you have someone acting in a release manager capacity that is either A) all about the requirements or B) all about giving the customer everything they want regardless of cost, you're in trouble.

Train people in this role how to help you out, provide air-cover, and also how to serve the customer. If you are the guy/gal responsible for delivering the product find someone who complements you very well and understands how to communicate, deflect, and the like. I'm not yet sure if the Release Manager needs to be technical in a software project but I believe the answer is definitely yes, since if they aren't they can't deflect and backlog as necessary, that would end up falling to you.