http://www.xantrex.com/documents/Backup-Power/powerpack-400/XPower%20PP400%20Plus_DS20040824.pdfI used one of these to power my booth at fairs and swapmeets when no power was available. Electronic cash register, halogen lights plus a public address system all powered for up to eight hours.
You would need to determine how much power the Cpap uses and then calculate how long something like this would last. There are more powerful versions or you can use several batteries in parallel and connect an inverter to make your own. Recharge it with the generator.