I have been able to run a Vector 20a charger at 20a that wants 320w for input from a 400w MSW inverter (rated for 320w continuous) clamped to the truck battery. Of course you want the engine running so the truck battery does not go flat.
Even with the hood up, you need to keep the inverter away from the engine (not a problem for the OP) because the heat triggers the inverter's heat shut-off. Hang it out down over the fender.
This was a test for if my Honda gen didn't work (never happened yet) and not intended as a regular procedure.
