The 12 volt DC to 120 volt AC inverters are typically square wave output. Commercial AC power is sine wave. A square wave has lots of harmonics (higher frequency noise) that needs more filtration than commercial sine wave power. This interference shows up as video noise and audio noise. If you know somebody who is experienced with electronics, they might be able to help you build a filter. I don't want to attempt that at a distance, because you can be severely killed by 120 volts AC if you don't do it right.