DootDoot,
Indeed the TPL timing is only approximate and requires external resistors. It's perfectly fine for approximate interval reporting.
The RV-3028-C7 is a great little device. But you will find that even at 1ppm it will drift eventually. You will need resync with the base station.
I get this question a LOT actually, people think that each node needs an RTC to keep accurate timing for some reason. I can see some situations where an RTC per node is absolutely necessary but IMO in most cases it's not.
I propose a different approach which requires no extra components. It has been demonstrated in practice and it works very well. Let the "RTC" be at the base station. Let the base synchronize all the nodes as they come online. The base can tell a node if it's too far off from its time slot, where that is required. This way you can orchestrate a large network of devices with extremely good precision in terms of when they wake up to send a message. Then the crystal drifting variable of the nodes is eliminated.