How can I get a unique address for all my Arduino boards while in RS485?
Just assign every node a variable like "const int ID = 5" and so on and then before sending the instructions send a line terminator, the ID, line terminator and the data you need to send to the nodes. Every device listens but they all wait their ID to accept the commands. For example:

Serial.write('\n');
Serial.print(ID);
Serial.write('\n');
Serial.print(|LT2|LP3|LM5|LR4|LI6);
Serial.write('\n');


NOTE: This is the easiest way. Most likely not the most reliable or optimized.