Having logged many, many miles in warm weather in a 24' C I can tell you that the dash A/C is more than capable of cooling the unit while on the road.
The Ford A/C in particular is actually more powerful than the rooftop unit in terms of cooling power, its drawback however is the relatively small fan.
The trick is to open the rearmost roof vent wide open and set the dash A/C to point straight down the aisle. This will keep the front seat occupants cool but not frozen. The vacuum created by the open roof vent pulls the hot air at the ceiling level out and draws the cooler air from the front to the back.
I've been doing this for decades. It works well.