The RTA allows you see multiple frequencies at the same time, which is its advantage.
Use a mic, run it into a volt meter or an oscilloscope, and a few hz at a time, pump a sine wave through your monitors. Record the volume level for each frequency, and you can graph what the RTA would see manually, frequency by frequency, then tune the room accordingly.
I prefer the RTA method, as "at a glance" you can get a better feel what the room is doing to your audio.