I haven't seen the articles that J. Walker is referring to, but traditional RV wisdom is just the opposite. Even if it is a break even, the roof air is providing a lot more cooling for the same money.
My diesel Onan 7500 only uses 0.3 gal/hour to provide a continuous 20A, which is enough to run one a/c and the fridge as well at a 100% duty cycle. If the a/c ever cycles off, the economy gets even better. The dash a/c always runs full out (even with the temp control set above Max Cool) and it draws about 15 hp from the main engine.
The last time I made a serious attempt to compare was with my1996 gasser and a 460 Ford vs a Onan 5000. The dash a/c cost me a bit under 1 mpg (a 14% decrease) while that Onan used about 0.4 gal/hour to run one a/c and keep the front of the coach cool.
We can't keep the front cool with just the dash a/c unless the temps are below 80, so we use the house a/c anyway.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL