Ok. So the first thing you need to be aware of is that the hardware serial port on the UNO (pins 0 & 1) are connected to the on-board serial-USB interface which is how your UNO prints to the IDE serial monitor.
As you need to develop the code, it will be a huge help to you to be able to use this hardware serial port for debug printouts so you can figure out what your code is doing and why it is doing it. This applies to both the sender and receiver UNOs.
Start with a software serial port (see the standard libraries for examples) on each UNO on say pins 2 & 3. Software serial ports can't go very fast so you are limited to a baud rate of around 19200 for your software serial port on an UNO, but I would recommend you start with 9600, or even 4800 baud.
You can simply connect UNO #1 pin 2 to UNO #2 pin 3, UNO #1 pin 3 to UNO #2 pin 2 and UNO #1 GND to UNO #2 GND.
You can then see if you can write a byte from 1 UNO to the other. Remember - start small and simple.
Maybe even read a character in from the IDE serial monitor and write it out so that the other UNO prints out the received character on its IDE serial monitor.
Have a read of the Serial Input Basics tutorial and make sure that you know how example 3 works.
What is the complete message format for the RCP? Does it have unique start and end characters in its response message like the RTIO message?
Get your RTIO UNO code working so that every few seconds it transmits the first 6 byte message.
Then get your RCP UNO code to detect that message (see serial input basics).
Then get your RCP UNO to respond to that message with a simple blank message of 20 bytes.
Then get your RTIO UNO to read in those 20 bytes and print them to the serial monitor.
Repeat the above for the second 6 byte message and the 8 byte response.
All this testing is done without using any RS-485 hardware.
Once you have it working, you can introduce the RS-485 hardware. The brief says it's a 4 wire interface so you will have 2 RS-485 modules on each UNO, one module in permanent TX and the other in permanent RX.