That gizmo doesn't look like a complete soft-start module, just the main component to reshape the AC waveform. Missing the control logic, the voltage and current measurement circuits and the microprocessor brain to name a few.
I'm guessing a large part of the cost for RV soft-starters is the manufacturers of such things feel their microprocessor programming is printed on gold leaf. More than likely only $20 worth of components inside one, if that.
2013 KZ Sportsmen Classic 200, 20 ft TT
2020 RAM 1500, 5.7 4x4, 8 speed
