I remember reading a post about someone who did what you are proposing, I was interested in doing the same. I remember that it involved 3 (I think) relays and a lot more wiring than I was willing to do. However, Maybe your AC is different, and yours is wired for a remote control.
If I can find the post I will let you know.
OOPS! Re-reading your post I realize I misread it. Forget what I said above.
Class C, 2004/5 Four Winds Dutchman Express 28A, Chevy chassis
2010 Subaru Impreza Sedan
Camped in 45 states, 7 Provinces and 1 Territory
I am guessing the Y is the cooling compressor, the two G's are low and high speed fan, and the B is 12V+/-. The other side of your 12v would have to come from the thermostat. Shouldn't be that difficult, just get a good digital thermostat with two speed fan control. Can you run new wire to the thermostat location from the A/C and from the 12V panel?
You will need a 12 volt supply to run the system, unless you want to hot wire(120 volts) the Fan and Compressor and use NO tstat. The AC has NO 12 volts unless the original tstat control board was left mounted to the evap cover. And IF so, the control board has no 120 to 12 volt power supply, you still have to find a 12 volt power supply. Doug