What is our cross-region DR solution with 0s RPO?

I don't know if we can "guarantee" a RPO=0, (think 11 9s but not 100%). However, you can layer different approaches together to get close. Starting with standard approaches in RDS such as multi AZ, and read replicas helps get us closer to RPO=0. After this you can start thinking about the design of the entire workload. A good example is https://aws.amazon.com/blogs/publicsector/a-pragmatic-approach-to-rpo-zero/. You can even add more tools to your pipeline to add redundancy to capture data that is inflight such as SQS for example. You have to be careful, of course, not to over engineer the solution but that depends on the customer's requirements.