I didn't read the whole thread in detail but it sounds like you have 2 choices.
1) Get an inverter of sufficient capacity and run 120V hot and neutral wires to standard AC outlet(s) near bed. Wires should be sized to max current capacity of inverter. Adequate fuse or breaker is required as well (on both DC and AC sides). Wire should be run in protected area or commonly available sheathing. As others have said, stranded wire is recommended due to vibration. Also, for automotive use under the chassis, wire with correct insulation type is recommended (TXL, GXL or SXL).
2) Get the correct DC-DC power supply for your CPAP(s). Run a single AWG 12 "hot wire" to a quality 12v connector near the bed. Connect the ground side securely and electrically to the motorhome frame. Fuse or breaker near the battery (probably 20A or 25A for your two CPAP machines). EDIT: if your 12V fuse panel is anywhere near the bed, run a wire from spare fuse to two 12V connectors as BCam did. I could not easily access mine to run another wire.
I did exactly #2 (except I ran AWG 14 wire for ONE CPAP) recently and had no problems with my two 12V Interstate Deep cycle batteries running the CPAP all night, and sporadic use of the LED house lights (you have converted to LEDs for boon docking - right?) without running the genny. We did power up the genny for a couple hours or more during the day to recharge, and if it got cold, to run the furnace fan, which would quickly run the batteries down.
The 12 to 24V supply for the Resmed S9 includes a quality 12V connector and on/off switch. Your CPAP may very well use the same unit - not sure. I cut the battery clamps off the supplied connector assy and spliced my 12V hot and ground wires on using crimp type butt connectors. I installed an in-line automotive fuse holder near the 12V terminal block in my battery compartment, and from there ran my single AWG 14 TXL wire through 1/4" split-loom wire sheathing and used wire ties every few feet to secure it to the undercarriage of motorhome. I drilled a 1/2" hole in the sheetmetal of the storage compartment under the bed, the lower part is the rear hatch with spare tire, etc. The upper part, with luan floor, is the storage compartment under the bed. I passed the single "hot" wire inside split-loom thru that hole, then thru another 1/2" hole in the luan. For ground wire, I drilled and tapped a hole in the square steel tubing frame holding up the bed, (which is welded to the motorhome frame). I scraped the black paint in the area around the hole and put in a 10-32 screw to secure a crimp ring terminal. Routing and entry of yours depends on your motorhome design. You may have to crawl around under it and possibly take some measurements to figure out where to run and how to penetrate to inside safely, and how to connect the ground. I used some sikaflex caulk I had laying around to re-seal around penetration hole. I drilled a small hole in the side of the bed frame covering (vinyl covered hardboard) for the supplied 12V connector wire to just hang from. Later I will make a proper bracket and mount the connector inside the under-bed storage space, with only the end of the connector and the switch, surrounded by a small plastic bulkhead on the outside. (I may add an LED and resistor as a power-on indicator).
You will need wire cutters, a crimping tool, drill and bits for either approach. If you run 120V AC, extra caution should be taken to insulate/seal any connections. In my case, I also used a thread tap for the ground connection screw in the 1/8" thick tubular steel.
Definitely do the calculations for adequate battery capacity. Don't forget other stuff you might have on (lights? radio? fan?) during the night. Note it is recommended lead acid batteries not be discharged more than 50% capacity to prevent damage (significantly shortened life). If your batteries are inadequate, consider getting a small generator with remote start - you could put it away from the RV and start/stop it at night from inside.
The 12-24 DC-DC CPAP supply will very likely be more efficient that an inverter and standard wall supply so if your battery cap is on the edge, it would be a better solution. Pick your poison and good luck.