I have two Group 27 12V batteries hooked up in parallel that are dedicated to CPAP use. I can get 4-5 nights of usage running off an inverter WITH the heated humidifier in use. This has worked for over 10 years for me. The batteries are also hooked up to a 3.5A Smart Charger so if I am running the generator it will help to recharge the batteries(or at home between trips hooked up to shore power).
The OP should have no problem with what he has stipulated in his question to the forum.