To answer What you asked ( I think ) when I bought our marquis the dash air didn’t work and I was quoted Over a thousand to replace the compressor and the dryers etc serviced. Knowledgeable people said it wouldn’t keep up and I would need to run a roof air . I let it go till later. It’s now 6 years “later” and I just run the generator and the roof airs ... I’m plenty cool while driving and I’ve now spent 2 summers traveling in Semi hot areas
. Dash air is not more powerful than the roof airs or even one large roof air . it does blow cold in your face I suppose I see that as an advantage. I can see closing off the front with some curtain contraption would keep the cab cooler on dash air alone. My question is when you get to your camp and open that curtain your met with a wall of heat and it will take time to get it back down. Why do it ? The bedroom will get crazy hot with the diesel just under the floor.
I guess folks do all that to save a few bucks in the generator use. i prefer to be comfortable all the time In The whole unit . the roof airs do that. Just my take...