The description can be reduced to numbers.

Over the Road Air (OTR) is a whole coach AC system that is about 80,000 + BTU. It is used while driving since it is only engine driven. It is extremely effective. Only Liberty consistently builds and sells coaches with OTR, and a Liberty without OTR is very rare. Other converters will buiild or have built conversions with it, but they are far less common.

Those that build conversions without OTR justify that on the basis of it saves bay space (in actuality only the equivalent of 1/2 bay, if that), that their coaches can run house AC units from an inverter or engine driven generator, or they install a system as Dale says that has less output. The reality is that converting a coach with OTR is a challenge. The converter not only has to provide ducting for the OTR, but the air flow has to be balanced making an interior design an engineering challenge.

Without OTR a coach will typically have 3 or 4 Cruise Air units, 3 or four roof airs, or a couple of "package" systems. The Cruise or roof airs are likely to be around 13,000 BTU for 39,000 to 52,000 total AC capacity. The packages are likely to be around 26,000 BTU for a total of 52,000. The short version is that OTR is significantly more capable than anything else.

As long as the coach temperature is maintained it is likely the other systems will be adequate. Where problems arise is when the coach is heat soaked and the owner wants to cool it down. It will take forever. If the coach has OTR an owner can run the engine and the other AC units and in a very short period of time the coach is cool.

There is no way Cruise Airs will cool the coach in the desert southwest when driving down the road. Heat radiating from the road surface will not allow the condensing units to adequately cool the hot freon gas and they will shut down. In this instance roof airs will be the better choice. The arguments about which is best are all over the place. Those opposed to roof airs talk about the noise level. Those opposed to OTR talk about lost bay space or the fuel and HP required to run it.

Personally I will not own a coach unless it has OTR. It is at the top of the "must have" list. I will look no further once I see a coach does not have it. Others are likely to have the opposite opinion.