Caravan earth

[canis]

Pure sine wave inverter producing mains voltage, mains fusebox with RCD, both earthed together. Connect to caravan chassis or not? I say not, my client doesn't agree and is insisting the chassis be earthed. He's even talking about carrying a metal stake around with him to hammer into the ground at every pitch.

Am I missing something here? Since the power supply is earthed to the mains end nodes, then connection the the physical Earth (the wet rocky orb floating in space) is not needed, right?

As for connecting the aluminium chassis to the inverter ground, won't this have some wierd effect on the vehicle -12vDC?

Can anyone with a few more brain cells than me solve this?

Thanks very much.