Can I run my cpap on a 400w inverter without idling?

well I am an EE, so I get what you are asking. I recommend getting rid of the inverters when ever possible and they make 12 volt "power supplies" that can be charged by a 12 volt source while being used. I think some of those battery power packs to jump start a car can be used in this manner - there are so many of them out there I don't know which one to point anyone to.

If you go and put in one of those computer backup supplies, it just adds some complexity into the mix. The output of those are pure sign wave or should be but they may not work with inverters where the signal is dirty.

I would go the 12 volt path like i said, I understand that most newer cpap machines have 12 volt power supplies available and I think there are a few generic ones out there, use that, eliminate the inverter as much as possible (they do you power to do the up voltage conversion) and get some battery system that takes some power (not full power) from the truck batteries as needed to keep it going.

I just checked mine, it uses a 24 volt 3.7 amp power supply, rated at 90 watts so for 12 volt operation, that is 7.4 amps that are needed to run it with the humidifier (mine is built in) but just running the pump and only the pump, I think the power consumption would be close to 2 amps at 12 volts.