Lets say your engine is running at 2500 RPM.
You plug in one of those DC powered tvs to the lighter jack and turn it on.
The car is still running at 2500 RPM driving down the road at 55mph.
There was NO EFFECT on the mechanical load of the engine, therefore MPG was not effected.
If you increase the electrical load on the alternator beyond what it is rated for (you will probably blow a fuse before this happens) your engine will eventually stop running and your battery will be dead.
I refuse to drive a car that modifies the RPM of the engine according to the electrical needs. F--k THAT!
Within the alternator's rated capacity, if you perform the above steps, switching on the TV will increase parasitic drag on the engine. The drop in rpm will be barely noticeable, because the engine control opens the throttle to compensate - as if you were climbing a hill. You're holding steady at a given rpm, but you're burning more fuel to maintain that rpm given the increase in load.
I'm suprised that after nearly 200 posts someone has not mentioned the 480 system that I have on my car. Four windows down and 80 miles an hour.