How can I create a moped DC source from the alternator using the chassis as common GND for both AC and DC?
If you want to ground both the AC and DC, then the easiest option is a voltage doubler.







simulate this circuit &ndash; Schematic created using CircuitLab

One half cycle delivers power to the output. The other half cycle recharges the flying capacitor. Use suitably beefy diodes, not the default 1N4148s shown there. Use large enough capacitors for your frequency and current requirements.

Don't want double the voltage? Tough, use a buck converter to drop it to what you actually need.