Changes by year

Does anyone have any information on what upgrade/improvements TM made as the year models evolved? i.e., What year did they transition from wall a/c to roof, year of power roof vent, when did they incorporate the pre-wiring for radio/cd, etc? I have seen many pics of older TMs and they don't seem to correlate to later models. This would help prospective owners decide whether they wanted an older model or newer. Thanks