So being a long time CPAP owner and user, there are various options:
1. If you don’t need/use a humidifier, then you can get a 12V adapter for most CPAP machines and it can run off your battery.
2. if you need to have a humidifier running, I believe you are stuck needing to run 120VAC.
That being said, #1 above is easy enough to do there SHOULD be a few 12V outlets in your RV that you can plug into, although they are not in every RV.
As for #2, you will need to look at the power block to see what it pulls and you can purchase a 12VDC -> 120VAC inverter, I would recommend a pure sine wave one(they are more expensive but better for electronics than the modified sine wave ones). Depending on the needed draw per hour and how long you sleep, you can possibly look at things like Uninterruptible Power Supplies (UPS), I run mine off of one of those so if electricity goes out, the machine doesn’t try to suffocate me 
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That is the 101 in using a CPAP in an RV, want more specifics, feel free to contact me. I need to get this info into a blog post as it gets asked quite often. Hopefully that at least gives you some ideas.
my contact info is: 740-206-8522 and info@rvmentor.com