The dash a/c in our MH works great however, it's nowhere near enough to cool down a 39ft MH. It's basically just for driver/passenger comfort while driving. On hot days we always fire up the Onan and run the rooftop a/c units as well as the dash if needed. Usually we don't need the dash once the rooftops are on.
Fuel consumption is really not that bad. But hey... when your only getting 8mpg to start with, whats another half gallon per hr lol.