CPAP: I use two small 12 amp/hour gell cel 12 volt batteries, connected in parallel (12 volts, 24 amp hours) in a sturdy carrying bag that I leave "on charge" using a "battery minder" smart trickle charger. I connect my ResMed power supply/converter to the battery and to the ResMed CPAP. I don't use the humidifier, which gives much longer battery life. There are many other "travel power" battery systems that will power your CPAP when 120vac is not available. Normally you will have 120vac when your RV is connected to "shore" (camp) power.