We have same situation, wife uses CPAP. We have not tried it yet but I think two fully charged RV batteries will get through the night but I would recharge next day. You can buy a 12 volt battery pack for CPAP, many sources available, one from our CPAP company is $200, kind of a lot. Amazon has various 12 volt battery packs for less. Please let me know how it works out and what you end up doing. Good luck.
Emory in MD