[cabmandone]

If it's there motor on or off might be a bad inverter or just a bad inverter ground. Ground is cheap, if not easy place to start. Proper size of ground wire makes a difference too.

Side note: I'm not familiar with RV head units but if the noise is coming through the stereo system, I'd start with where it gets ground. Could also have a power and audio wire run too close together and you're picking up a hum. But normally that one will vary with the rpm.