Lots of options for stand-alone power supplies for CPAP.
A few examples.
If they want to boondock, fundamentally the goal is to get through the night without a generator or solar charging without draining the house batteries below about 50%. Like the house batteries, these CPAP batteries can be recharged during the day.
If they plan to boondock a lot, size a solar array to handle the following:
a) climate...how much sun do you get?
b) battery bank...the solar array should be able to FULLY charge all batteries by sunset.
c) all daytime loads...if you use power during the day, that counts against the solar power charging.
d) bad weather.
e) backup generator.
In very crude terms, one x 100 watt panel per battery (including CPAP battery) in sunny climates. Double that in grey climates (Pacific Northwest).
Your RV battery bank might be able to handle the CPAP load, but can it also handle the furnace, pump, lights, entertainment, and, and, and overnight? Since the CPAP is a big load, this is where the external supply can make the difference.
For sure, a
big generator can recharge all batteries during the day. But listening to a generator drone on all day is no fun. Go solar.
A
400 watt solar system is probably adequate for most applications...and costs the same as a generator. Windy Nation is another reliable brand.
Add a 120 volt inverter to charge your CPAP batteries...assuming they don't have their own 12 volt inputs.
This solar setup is totally set it and forget it. It connects directly to the RV battery bank in parallel with the RV's converter/charger. No muss, no fuss. It's always charging when the sun is out...towing, stored, parked, setup, all the time.
I have a 100 watt system on my PUP charging a single group 24 12-volt battery. I'm careful what I use, but I NEVER run out of battery power.
Solar is not an infinite well of power, but a properly sized system and judicious use of power can achieve a balance where you never run out of power for your needs.