The problem with using the dash a/c to cool the m/h is not the cooling capacity of the unit, the system used in the Ford chassis is capable of (depending on RPM) producing 50,000 BTU's but at highway speed, about 30,000 BTU's. that is more than double the capacity of a typical rooftop unit.
The problem is the relatively feeble fan to circulate all that cold air.
If however you open a roof vent at the very back, like the bathroom, this creates a powerful vacuum drawing the warm air back there out, and cooler air from the front back to replace it.
In our 24'er we can keep the entire m/h comfortable enough doing this even if the hottest weather as long as we're moving of at highway speed.
