The CPAP machines are ResMed Mirage S9 Series. We do use the humidifier. The machines do not run directly off DC power, they have an AC cord attached. The label on the machine states it is 24 volt with 1.25 amp (AG) and 3.75 amp (Sys). The batteries I want to use are the house batteries which are wired in parallel. I have a total of 420 hours in Reserve Capacity between the two batteries.
You need this to run off of a twelve volt system.
http://www.cpapsupplyusa.com/36970-ResMed-S9-DC-Converter.aspx
Install a cigarette lighter adapter plug near your bed, make sure you use a large enough gage wire so that you don't have power loss. With this device you can run off a 24 volt supply or 12v supply, heater or no heater. Power rating for appliances can be misleading, many times they label the appliance for way more than they actually consume. 3.75 amps is a small power drain, but your usage is more likely half of that, perhaps 1.5 amps. A pair of healthy interstate RV/Marine batts hooked in parallel to provide 210 amp hours at at the 20 hour rate. At your stated load they will last about 37 hours and leave you with 50% discharge (you should not discharge your batteries to less than 50% very often, you should never go below 20%) Your appliance probably uses much less than 3.75 amps, but lets say that it uses 2 amps. Peukert's law calculates your battery run time at 79 hours and 8 minutes and still maintain 50%.
The upshot is that you should get between 4 to 8 days from your system, depending on how accurate your advertised amp draw is.
Doing this correctly will prevent you from running a generator, and make it more feasible to run solar. If you want to know how the Peukert's law works, I can send a link.
Good luck
PS reading your post more closely, I see you have way more amp hours available and you have two machines. I will work on this a little and post the calculations again. But you need that charge cord adaptor that allows you to un off of 12v. See the link.
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What kind of batteries do you have? I see where you say you have 420 hours of reserve capacity, but that is not enough information.
Tell me how many batteries and what kind they are, and I will tell you how much capacity it will take to run your appliances for 8 hours of sleep.
example, you have 420 amp hours, only 50% is usable (battery life diminishes after regular discharges beyond 50%) so you only have 210 available at the 20 hour rate. So you can see how this gets involved.
So if you could provide me with the number, brand and style of batteries you have in your rig, and I will get you a figure that you can work with. See the link in my previous post.