The cab of a Class C is not the same environment as the cab of a Class A or B. The cab of a Class C has way less heat gain because it's completely shaded at all times by the cabover. What this means is that a large amount of the cab A/C's BTU capacity can be used for cooling of the coach area of - at least - Class C motorhomes as large as our 24 footer.
With our E450's excellent Ford designed doghouse insulation, heavy floor insulation as installed by Winne, our cab vent fan on it's highest setting, it's air circulation mode set to Max A/C for complete recirculation/reprocessing of the interior air, and it's fan outlet vanes aimed over our shoulders right back towards the coach area - the coach DOES remain comfortable in scorching outside temperatures.
As I've mentioned in other posts, I think a key factor may be using the cab air of a Class C in it's Max A/C configuration to stop having to constantly cool new outside air. That's how the rooftop A/C handles the air and how home air conditioning systems usually handle the air.
In hot summer temperatures we know the above procedures work because we set things up this way all the time when we have passengers back in the coach area when traveling, and even when we ourselves sit at the dinette in hot blacktop parking lots eating lunch with the V10 idling and it's cab air set up as I describe above so as to not have to run the rooftop A/C and Onan generator in noise-sensitive parking situations.
