We had a "long" discussion already about RTSP, and somewhere in the middle you can find a flow I created to achieve rtsp decoding in Node-RED. But it doesn't always work (as @SuperNinja reported me), but I have not enough Ffmpeg knowledge to get it solved, and no time to experiment with it...
But I think that flow might be a starting point for your question...
I have planned to continue improving this contribution further in a couple of months. But if I'm not mistaken, Onvif only offers you the ability to get an rtsp url from your camera. But it doesn't do any decoding. So my Onvif nodes won't be of any help to you for this particular issue you have.
