Hi,
The jetson_multimedia_api samples is to demonstrate hardware functions. The input is h264/h265 stream. For using jetson_multimedia_api you would need to implement code for depaying RTSP stream.
For using OpenCV, an optimal way is to use CUDA filters. It can map RGBA buffer instead of BGR. Please take a look at this sample and see if it can be applied to your use-case:
Nano not using GPU with gstreamer/python. Slow FPS, dropped frames - #8 by DaneLLL