wbwood wrote:
The fuse panel, converter and all are under the bed. Putting a plug in that area would be ideal.
If you have an empty "extra" slot on your 12V fuse panel, just run a wire from there, put a fuse in, and put in a lighter socket at a convenient location. Running the wire should be pretty easy. You may have to remove the mattress and unscrew the deck of the bed or something along those lines to get access to the space.
If you don't have a spare 12V fuse location, it would require a tiny bit more fiddling and/or thinking, either tapping into an existing circuit that would not be used at the same time as the CPAP machine (or at least would not have a significant current draw) or possibly putting in an auxiliary 12V fuse panel.
For a load the size of the CPAP machine, there's no great advantage to running wires directly to the battery rather than the 12V distribution center assuming the DC wiring is at all reasonably set up.