According to my d/s, RSSI is a PWM output. You can filter this using a simple RC low-pass filter, and feed
to an LM394 or LM3915. They apparently still make these things.
http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_24230_-1
http://www.jameco.com/1/1/2533-lm3915n-1-lm3915-dot-bar-display-driver.html
For ckts, do a google image search on LM3914 or LM3915,
http://www.google.com/imghp