Since the generator is more efficient at producing power there have been strong arguments that the amount of fuel used by the generator to supply power to run the rooftop air is less than the drain on mpg from the engine having to run dash AC compressor. And I would like to see some statistics that the output of the dash air is greater capacity than two rooftop AC's. I struggle to find reasoning behind the logic of not using your onboard gen and Ac's, that's why they are there. If the dash air was more efficient why do you need a generator at all? Why not run the coach engine and the dash AC? The answer is simple - the dash AC is not better at cooling the coach, the rooftop air is. I know personally that when travelling in temps above 85, without the rooftop airs it would be downright miserable. No way the dash AC can cool the whole coach. Spend all that money on a MH with all the conveniences and not use them seems silly.
To the OP, this is simple. Yes you can run the genny and rooftop airs. Most folks do it all the time. The impact to MPG isn't worth the time trying to do the math. Do what makes you comfortable.