You didn't say how many batteries you want in your array. 2? 4? Size?
My first impulse would simply be to weld a bit of plate into the existing battery box location to extend it wide enough for double the space. If that is not enough, I would go to the back bumper weight. The 1007 isn't particularly tongue heavy, so an extra battery would be fine there. If you need two sets, then the back bumper would be okay, but they will get very shaken back there.
If you want to go inside, you might consider increasing the dead space inside the step ups to the bunks, You might be able to eke out enough space there for batteries that is not currently used, but your step up would increase. There are lift cables in there too though (only one cable in the front box), so it might not be feasible.
Also, have you checked with your CPAP manufacturer to see if they have a 12v adapter for your machine? Most of them don't actually run on 120v, so converting 12v to 120v so that it can be converted back to 12v (or less) is highly inefficient.
2011 Jayco X19H (purchased 2015)
2008 Jayco 1007 PUP (purchased new, traded for the X19)
2018 Nissan Titan Midnight Ed.