Check your CPAP power adapter, I will bet that it runs on 12V or 13V power natively. Therefore, you have a few options:

1. Get a 12V-110V inverter (most switch from car to empower port no problem). Your CPAP draws at most 50W (mine draws 25), so any inverter should be able to handle this.

2. Consider buying a battery pack next time you're in the US (they're cheaper) - such as http://www.amazon.com/Energizer-XP18.../dp/B002K8M9HC (not an endorsement, I use an earlier model that works quite well tho)

3. See if you can get a car adapter specifically for your CPAP. These are usually atrociously priced in the US (medical certification/liability/etc.), may be better in the EU.

Especially if you have the battery pack, you are now covered in just about any eventuality. If the crew give you grief, tell them that it's for their safety as well ;-) (I am a 10/10 on the snoring scale - they heard me two soundproofed rooms over ;-)