We have no idea as to what size the battery is in the power unit (AmpHour rating), so it would be difficult to determine how much time you will get out of the battery. I doubt very much that you will get 8 hours with the humidifier.
Based on my unit, with the humidifier running.. (only way I can use it), it uses between 4 and 5 amps. So, running it for 8 hours could use between 32 and 40AhHrs each night.
If you go up to the GOLD bar above and click on Search, enter CPCP and you will find a lot of member information regarding CPAP connections. There is one post about a 12VDC to 24VDC converter a lot less than $250.
I rewired my setup for using the TT batteries.
Don
My Registry
RVing with SOLAR