The (DC) fuses are for DC circuits--lights, the water pump, fridge controls, that sort of thing.
If you can plug your CPAP into a 12V receptacle, you don't need an inverter, just a 12V power point installed somewhere more convenient. That would be more efficient than running an inverter (to make 120V out of the 12V power) just to plug in your CPAP machine's power brick (to make 12V out of that 120V power), with each of the two conversions consuming some energy in the process.
As to why they don't put more 12V sockets in useful places, your guess is as good as mine.