Pam it is probably less expensive and gives more capacity to expand the built in battery capacity in your RV, and possibly upgrade the the battery charger that is built in (called a converter), or add a beefy portable battery charger (a big one, 40 amps).
Even if you do not want to add extra wiring to your camper, you could locate some types of batteries near the CPAP, charge them with a portable battery charger plugged into a wall socket, and then run the machine from them over night. Just be sure they are the kind of battery that can be used indoors.
Jim