AC vs DC
Just to add to the discussion:
On DC loads - Fine to Ground to the frame (although it may cause "noise" on sensitive equipment when loads include dimmers for LED lights or motor controllers), but in aviation we generally return to the ground bar (battery negative connected through low impedance) from all loads to reduce electrical noise.
On AC loads - DO NOT use the frame as neutral return on AC loads. It WILL energize your frame at different potentials thorughout and give folks a "HOW YA' DOIN'" wakeup when they grab the handrail while standing on the ground. All neutrals should go back to generator neutral or inverter neutral and should be switched between the two when changing sources. if AC neutral is grounded. it should only be grounded at the source, in one place, though very low impedance (short distance, a lot of metal) to earth ground.
It is OK however to Ground AC Ground to the frame, which then in turn is connected to the big "ground" lug on your inverter or generator... and ALWAYS connected to the pedestal ground when you are plugged in (earth ground becomes frame ground).
This will keep the wakeup calls limited and pacemakers working.