Trojan T-105 6-volt golf cart batteries are the gold standard for serious RV'ers.
But any decent 6 volt pair will be better than 12 volts, so I'd start there. Just ditch 12v.
Also, pro tip: Turn off your CPAP reservoir/hose heating features and the CPAP won't draw much at all.
Out of everything so far, this is the most successful route and there are simple explanations why, especially with an inverter in the equation. The T-105 is also a great battery, just get a fresh pair or make sure they're both fully charged (via a hydrometer).
I did get a chuckle out of the 27 "poor" 31 "best" guy....but for a hint, its a liquid to capacity issue. The ones with more lead, less fluid, all suck in this application, regardless of physical size. The heat generated by running output and a charge concurrently has a tendency to reduce fluid level and run times, then even more charging takes place. Honestly the entry level group 24 with the lowest output holds up better than most higher rated 24's, 27's, 29's, and 31's (the same thing as a 29), because they have less lead and more electrolyte. They obviously don't run as long out of the box, but will hold up better in this less than ideal usage over time.
Two 6V golf cart batteries in series are less prone to the fluid loss and increase the capacity, although when done with them a fluid check and a manual charge (verified by a hydrometer) should be standard service.
RV's and semi sleepers even many today, are charging when plugged to power, when the generator is running and when going down the road. The standard charge VOLTAGE is often too hot for Marine/RV battery applications, which is why going to two 6 volts is beneficial: they displace the greater voltage, heat and discharge better because of those two reasons.
If the run time isn't there, go to four of them, just monitor the fluid levels and charge (1275-1300 specific gravity, 6.36 OCV after surface charge).
