If all you want to do is get the fan to cycle with the compressor that is pretty easy to do. My guess is just feeding the fan off the compressor lead would accomplish that as the fan draw is not much. However, it messes up the fan control when you just want to run the fan.
I also looked at relays that use 120-volt coils and I think there is space for one, but same issue with fan operation without the AC so you would have to add a Auto/On switch like you have on traditional RV thermostats.
If it were not for the wiring issue of getting to a wall thermostat, going that route would be easy peasy as the saying goes. :)
