Battery Recommendation

1. Be careful shopping for Gp27 and Gp31 deep cycle batteries. True deep cycle batteries are hard to find in these sizes except for something like Trojans. Most so called "deep cycle" batteries sold are "Marine" batteries designed for engine starting. If the battery has a cold cranking spec it is not a true deep cycle battery.
2. One true deep cycle Gp27 battery will give you ~ 105 AH; one true deep cycle Gp31 battery will give you ~130 AH. Two 6 volt GC2 golf cart batteries in series will give you 220 AH at 12V. Only half these values are really available since you don't want to discharge a battery more than 50%. If you do consider two batteries the GC2s are probably cheaper, and the series configuration is better than a parallel configuration in all aspects except redundancy.
3. The larger battery packs just extend the time between recharges, but the total time required is determined by how much energy you have to put back into the battery. Your usage suggests 20-30 AH/day. The recommended regimen is discharge the battery to 50% SOC, then recharge to 80% every two days. Recharging to 90-100% gets very inefficient and long. Then fully recharge overnight when you get home.
4. My recommendation would be to start with two GC2s and recharge as you find necessary. The GC2s will handle the 2-3 day trips and you can leave the generator at home.
   

I have just became aware of B+B offerings. My general rule is to shoot for the middle. Lowest cost usually means lowest quality and highest cost usually means marketing BS. While I agree with Furpod on Trojan 105s, the B+B mid-level is worth considering. The next option up would be two GC12s, also available from B+B. Two GC12s in parallel would provide 300+ AH. They would violate my preference for series vs parallel, they are expensive, they are heavy, and they require mounting tray work, but nothing is perfect. For the ~300 AH solution I would prefer these to Gp27 or Gp31 solutions.

The addition of a CPAP just adds to the magnitude of the problem, but doesn’t complicate it. You will still need some recharge capability after a few days. BTW always run the CPAP directly from 12V if possible. DC to AC inverters just waste battery energy as heat. The only long term non generator solution is solar, but that comes with its own set of issues including roof space and large oak trees overhead.

As you increase battery capacity you need to also increase charger capability. For two GC2s Trojan recommends charging to14.8V at 28+ Amps. While the OEM converter on the RPOD is quite adequate for its intended use it is limited for fast recharging, primarily due to the small wire connecting the battery to the converter. If extended off grid stays are frequent consider a better external charger. At max charging rate the power is still less than 500 Watts so your generator is loafing along, just burning gas and making noise.   See these clips from the Trojan users guide.

So my recommendation remains start with two GC2s, watch the State of Charge (SOC), and recharge as needed. You’ll soon get an idea of your usage and needs and adjust accordingly.

Camp on!