Using a true Sinewave inverter with the CPAP is highly recommended. The one you purchased isn't.
Placing the inverter near the battery source and running a 110v extension cord to the CPAP is more effective then running a long 12 volt line or using a 12 v outlet in the camper. Also, it gets the inverter out of the sleeping area.
If inverter or battery capacity is an issue, shutting off the heat on the humidifier helps a lot. You can still use water in the humidifier with no heat.
I think my HMO insurance approves replacements at 5 years. Or, if I think it is a problem and I really, really twist their arms......