Since I don't know much about length of wire, gauge, fuse etc., I can look at this with an open mind! I'd try to run a wire from the converter, under the kitchen cabinetry and shower. All is pretty accessible, if I recall, by removing drawers and the panel in the bedroom. The 12V outlet could then install beside the panel on the inside bedroom wall assuming it is deep enough to accommodate the outlet. You should also be able to run a wire from the water pump through this same panel, but I don't know what would happen if the pump and CPAP are on at the same time.
The reading lights in the bedroom can also allow unobtrusive wiring if you route it in the corners and into closet and down to front pass through. Mount the outlet up high in the pass through so it's not in the way/protect the wires from being bumped off.
I'm not clear whether you're trying to run a 12V CPAP or an inverter. I used to plug a small inverter into the outlet by the TV and run a combination of 12 V and 120 V extensions to reach the bedroom. Always had plenty of power for the CPAP. A travel 12V machine would be a better solution for more than one night as the inverter will draw the battery down in addition to anything plugged in.
I can't remember what size my inverter is--3 or 400W, but it's just big enough it came with 12V plug as well as alligator cables for a direct connect. So, if in doubt, tapping directly into the battery would provide the most power. CPAP demands are quite variable. I never used heat on the humidifier and could easily get through the night even with the furnace running occasionally.
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