I saw figures for dash A/Cs from 4k to 48kBTU. I throw in the towel on that one.
Since I live in Texas, the last motorhome I rented, I just used both the dash air and the roof air with the generator running. It cost me about 1 MPG, but kept the complete rig decently cool in 100+ degree weather.
It is supposedly cheaper to run the generator and roof air. Also, I don't think the dash air would come close to cooling a 40 ft MH. It would probably cool the driver compartment ok with dash air but not the rest of the unit. Too much cubic footage and too little air volume.