This evening I tested my new home-brew vfo rotary knob.
I keep the RC2000 connected and use the COM2 to connect an "Arduino uno" board with a level converter to comply to RS232.
I connected a 16x2 lcd display (useful now for debugging, don't know if in future I will keep it).
I used a DC motor as rotary knob, it is not precise, but it could be a good choice (at zero money...).
I programmed an analog input ADC to convert the voltage level that goes through motor (connected one side to 3,3v ref and the other side to arduino analog input).
If I turn the knob clockwise the voltage goes down and viceversa, so frequency has been programmed to go down and viceversa by 0,1khz steps.