First buy yourself 2 industrial 12v deep cycle batteries before you do anything. Then install a solar array. if you do in 2015 you will get a 30% tax credit for doing it (people like to say its not true but it is). This will take care of 90% of your boondocking battery needs.
I run a 3000 watt inverter off 2 Trojan T 1275, 12v deep cycle renewable energy batteries. I ahve flooded lead acid, but I would buy AGM's they are easier to contend with.
On the subject of running appliances, it is always best to run DC appliances on DC. It is a waste of energy to make AC power just to transform it into DC again for a DC appliance. Most likely your CPAP and CPAP heater is DC powered and all you need is a DC chord. Most CPAP manufacturers provide a DC power cord for about 29 bucks, mine stays in my rig.
I wired in a DC female cigar lighter adaptor that was mounted in a switchplate cover. I found it at the RV store in my area. I bought 2 of them. I was able to mount the adaptor in the wood panel on my side of the bed, the wires tied into the DC wires under the bed, the wires fed my DC lights above the bed in the slide out. Since this was already tied into the DC fuse box, no additional wiring was required. Works like a charm.
When a battery has a small load on it is more efficient. Its like a gas tank, if you do 55 mph you will get maximum mileage out of the tank, but if you do 100 you get a lot less. Batteries are the same way, so the Amp hours at the 20 hour rate is a good place to compare you batteries to other batteries, it does not tell you how that battery will act when you are only pulling 4 AH. My 2 12v Trojans will run your CPAP for 8 days without recharge, easily.
Here is how it goes on my rig on a normal boondocking day. 4 hours TV, 500 watt home theater, and blu ray DVD, 15 minutes microwave oven, DC lights as required. Go to bed, 8 hours CPAP and CPAP heater. Batts at 90% capacity, completely charged by noon having gone through the entire charge cycle. no generator required.
IF my batteries would have dropped below 50% charge during the night my "auto gen start" would start and run the generator until the batteries were again at 90%. My generator has never auto started. Although the microwave migh t cause a start depending on teh state of charge the batteries are in.
If you are a good DIY, the generator is pretty much "plug and play." but be advised that the generator is very wasteful when it comes to powering a rig. But it does come in handy for running air conditioners.
a generator motor is only 80% efficient if that, it is inturn only 80$ efficient in making electricity, your charger is only 20% efficient at charging batteries. You can see where I'm going with this.
It actually takes a generator 6 to 8 hours to charge a RV battery bank. 8 hours of operation will burn one entire cylinder of LP.
I encourage you to look into solar you will be glad you did. Your campground neighbors will thank you for not running that generator.
See my signature for part numbers of the items I used. Mine was worth every penny, and it was worth even more when Uncle Sam gave back 30% of what I spent on it.
BTW search SOLAR on this forum and you will get all kind of assistance on the subject.
Even a small solar array will keep that CPAP going much better than a generator will, and you never have to worry about CO getting into your rig. Nothing says nighty nite like CO in your system.
I have purchased a 2015 Big Country 3150 RL that has generator prep. I was thinking of getting the Onan 5500 LP genny. Does anyone run them at night when sleeping? When going down south usually in the colder months we lay over in truck stop or what have you and would like to run the Onan for the wife's CPAP and the furnace etc. Is it doable and does anyone do this?