I have heard that myth many many times.. I do not believe it for a second.
Thomas Edison optimized the effiency of a generator at around 90% (Now that is just the generator portion we are not including the engine here) and likewise an electric motor.
So once the power comes off the Engine Drive Shaft.. you lose what 19%.
The DASH A/C the power comes straight off the engine, NO LOSS period.
Now we have the fuel needed JUST TO RUN THE ENGINE.. No-load.. so you loose perhaps even more when you run the Roof air.
That said. Dash air is not big enough to cool off a 38' Motor home or for that matter a 15' motor home. when it's 90 out in the shade you got to run the roof air's no choice.
