Inverter

Four 6 volt batteries connected in series is 24 volts, so double check that. You are probably on 12 volts. If you have the typical 6 volt 230 Ah battery, you'll have a 460 Ah bank at 12 volts, of which half, or 230 Ah, is usable. At 6.7 amps at 120 VAC for the machine, that is 67 amps an hour at 12 volts, or around 800 watts, which would need a 1,000 watt inverter but would deplete your battery bank in around 3 hours after inverter losses. Since everyone else uses these machines with a lot smaller battery with no problems I suspect that your CPAP current draw is wrong, or is already in 12v, so double check that. Electrical things use power, which is watts. Watts is volts times amps, so any time you say a current draw number you also have to specify the voltage, or it's confusing.

Brian