Get a power inverter to hook up to your car (relatively cheap at harbor freight) and run an extension cord to your tank so you can run a small powerhead for longer periods of time.
If more than a few hours you will have to start your car (outside/not in garage) and run the alternator so that the battery does not die. No, you will not do any damage to your car by letting it idle for long periods of time (far less wear than driving on the freeway for long periods of time) and very little gas wil be used. Not as efficient as using a generator but alot cheaper when you are in a pinch.