My Dometic is non ducted but has the wall thermostat which has auto mode. This mode varies the fan speed by steps and shuts off the fan when compressor shuts off. There's also low-med-high manual modes which the fan continues to run while the compressor cycles.
What brand of air conditioner do you have? I would think something could be whipped up and retrofitted no matter what you have. Perhaps the fan and the compressor could be wired to run/not run at the same time. But I do not know the limitations of the relay/thermostat for this wiring configuration. Maybe someone here does know this....