A 750 watt inverter is plenty, just ensure you get a pure sine wave unit to ensure spikes dont harm any of your electronics. If the battery is new, check what size and type it is, and add a second matching one. Your CPAP will use 40 or so amps per night a little more if you use the humidifier, maybe a little less if you dont. With 2 batteries you should have 150 to 170 amps available so you wont be pulling the batteries below 50%.
As you know you can charge the batteries from your generator, but be advised, it will likely charge it to 85-90% fairly quickly, but getting the last part takes a long time, which is what solar is great for.
Good luck and enjoy the journey.