Basically an insulation monitoring device (also called RCD residual current device when used in mains power supply) doesn't necessarily need a transformer respectively an insulated supply circuit. A differential current sensor would be sufficient to detect a leakage. A DC/DC converter however offer the advantage, that you can still operate the device in case of a minor leakage without fearing electrolytical corrosion. And by applying a test voltage, you can detect shorts of circuit ground to earth.