As you may know inverters are very noisy. I learned through experience if you keep your ground wire short under 2 feet and it's gauge at a minimum of the gauge of your largest power wire you should have no problems. As an example the ground wire connected to my inverter is 6 gauge.
The ground wire should be connected to a solid member of the vehicle frame. If I recall unibody framed vehicles have very poor grounding sources and it is due to the periodic welds throughout the frame that causes grounding issues.
You might be able to find some pretty solid ground sources in the engine compartment just do not depend on things like the body to act as a good ground.