Originally Posted by pitman44
My DW uses a CPAP. I'm not sure how you'd run it off 12vdc. Is it one that's designed to use 12vdc out of the box or a modification to a 120vac unit?
Most (not all) CPAP equipment operates on 14 +/- 2VDC (12-16 volts). Almost all machines plug into 120 VAC outlets, and have a "brick" that converts that 120VAC to 13.8VDC for most machines. Many brands do have a DC/DC converter that will plug into a 12 volt RV or car electrical system and "condition" that voltage to meet the input voltage requirement for that specific brand equipment.
So, while your DW's CPAP plugs into the wall outlet at home, that "brick" converts the wall voltage to a DC voltage that powers her CPAP. If you look at the "brick" it should have a "nominal output voltage" listed on the tag. That would tell you whether you can operate it in your RV with "just a straight power cord" or if you'll need a DC/DC converter to power it from the RV battery.
That said, many (maybe even most) have a small 100 watt inverter that they plug into the RV 12 volt system and just run the CPAP off the 120 volt inverter output, which powers it just like they do at home.