5,000 watt 10,000 watt surge Pure Sine Wave Inverter

I'm aware of modern design, but you cannot get around the fact that several hundred amps of DC is going to require large conductors, ideally copper. I don't know that you could even connect 4/0 cables, to say nothing of the 750 mcm which would be required at minimum.
I say you would be lucky to get 1/2 the rated output.

As to country of origin, I trust that as much as I trust the specs.