This is a guess...
Perhaps the Magnum uses a frequency sense circuit to protect the inverter/charger from under-over frequency. Power supply or not, severe over-under frequency can indeed fatally harm such a device.
If the inverter fails to accept stepped down 220/110 volt power delivered via a step down transformer, then my (educated) guess is that would "Be The Ball Game". Mission hopeless.
Similar to twiddling with a gasoline engine to successfully run on diesel fuel. Not likely.
Either stop now, or stop after a giant cloud of blue evil smelling smoke assaults the nostrils.