Modbus RTU controller monitoring with arduino and RS485 module
To send series of bytes over Serial or any other Stream (Print) implementation use the "write buffer" method size_t write(const uint8_t *buffer, size_t size)

For the case in question:

byte request[] = {0, 0x03, 0, 0, 0, 0x01, 0x85, 0xDB}; 
RS485Serial.write(request, sizeof(request));


If someone is curios about the Modbus RTU request bytes. They are:


station address
function - 0x03 Read Multiple Holding Registers
register address - 2 bytes
number of registers to read - 2 bytes
checksum - 2 bytes


The response:


station address return
function or error code
data length
data - length bytes 
checksum - 2 bytes