...
According to SiK Manual, RFD900 has:
TX pin that is UART
which protocol?
Digital I/Os
GPIOs
Digital
ADC
PWM capable
“Mavlink radio status reporting”, including local RSSI and remote RSSI
So the RFD900 can use Mavlink protocol for RSSI reporting
AT commands (use RT commands for remote modem):
you can send AT commands to get RSSI report
Ardupilot implementation reference
...