Hi mczajka,
In the hybrid, if you use a 12 volt electrical load, like the ac outlet, headlights or radio, with the key in the run position, then the high voltage battery will keep the 12 volt system at nominal voltage. When the HVB gets down low enough it will automatically start the engine and recharge to 50% soc. So yes it will use gas from time to time. If your whole load is the 150 watts that the outlet can deliver, (and the energy in the HVB between 50% and 10%, where it starts the engine, is about 500 watt-hours) you can use the outlet for a couple hours between engine starts.
(you know Plug In Day is for cars that plug in to an electrical outlet, not cars that have an outlet - right?)
-mort