I've been able to do it using s-video port
xrandr is what you might need
For starters, try typing xrandr in a terminal and save the output to a file
then plug the RCA cable to the computer and to the tv, make sure the tv is turned on
then run xrandr again, and save it to another file
compare the 2 files and see if you can spot anything different.
If you do, then xrandr will work for you, we can proceed from there