What brand and model CPAP?? When I got my last one, I made sure that it would work on 12v. The previous one had a 12V input jack, and the newest one has a power block for 110v, and direct 12v input plug. I had a 12V power point installed beside the bed, fused especially for the CPAP. It runs overnight for several nights off two coach batteries.
Added on edit: My humidifier is attached to my CPAP. I fill it with water when parked, but turn it off. In my case, added humidity is not necessary so it works for me, but might not for others. Caution! always remove water before travel!