I have a 3kw DC to AC inverter. The enclosure of the inverter has a "dc ground" screw on it that the manufacturer says should be tied to the "dc ground in the system".

My "system" is a box on wheels that contains a 12v AGM battery and the inverter along with some outlets attached to the outside of this box on wheels. There is no vehicle chassis and there is no "earth ground" that I can tie it to.

Assuming the purpose of the DC ground nut on the inverter enclosure is to bleed of current should the enclosure ever become energized (so that the breaker can trip), would it be sufficient to mount a copper plate to the inside of the box on wheels and screw the ground wire to that? Would that not be the same principle as attaching the ground wire to a vehicle chassis (albeit with much lower mass)?

How could I test the efficacy of a solution like that?

Thanks in advance!

I would agree that the intent is to tie the chassis of the inverter to the chassis of the vehicle, which in your case is either (a) the box (if it's made of metal - is it?) or (b) the metal plate that you propose. So you solution sounds OK, but please confirm that the negative terminal of the battery is also connected to same plate.