Welcome to the forums.
I use a cpap machine in my RV. The first thing I did is get rid of the humidifier (it takes a lot of electricity) and then got a 12V plug for it. It now runs natively off 12V DC without all the losses of an inverter to step up the voltage to 120V and then the machines power supply stepping it down to 12V.
Is this the only thing electrical you are going to use in your vehicle? No phone, laptop, lights, refrigeration, etc? All of these things need power. The more of these that you can get a 12V power supply for the better. Don't plan on using any 120V AC household appliances for anything. There are other ways to accomplish the same task. The best way is to set a power budget:
cpap = 12V DC @ amps
laptop = 12V DC @ amps
etc.
Only by knowing how much electricity you need can an educated guess be made. You are in the rainy PNW so you will need a lot of solar for a small amount of charging.