I've been using a CPAP for about 20 years. :E
Chances are your CPAP will run on direct 12 volt. Contact the provider & get the 12 volt adapter cord for your specific model. It probably did not come with one, as they are sometimes an option. This does not require an inverter.
You have two options: :)
#1 If connected to shore power @ CG use that as power source.
#2 If not connected to shore power, use the 12 volt adapter cord & plug into any available 12 volt coach receptacle. If you are not using a humidifier, the unit will draw about 3-5 amps / hr.
Keep the coach batteries charged & you will not have any problems.