Hi,
I see that you want to put TCP/IP data into a slip packet in HDLC layer and transfer/receive via UART port instead of Ethernet port. Regarding the LWIP documentation, we have the example project and provide the source code in KSDK_1.3.0, unfortunately, we do not provide the documentation, because the LWIP is developed by third party, we only modify the low level part which is related to the ethernet hardware module.
Regarding the RTCS, it does not provide the slip protocol, but it provides the PPP instead of slip protocol. we have the doc to explain how to use the high level api function and the example code as a client/server.
The LWIP is widely used protocol, I think you can get the explanation/description by Google although the NXP do not provide the doc, furthermore, the LWIP provides the slip code exactly. I think the LWIP architecture is more clear that RTCS, I suggest you use LWIP as platform.
BR
Xiangjun Rong