re: "have had a hard time because of the complex relationship between draw rate and maximum draw
" -- this is the Peukert phenomena.
The 20 hour rate usually used to indicate battery energy capacity is about a 60 watt draw for a typical RV battery. For 2 batteries, that'd be 120 watts.
And do use watts for power
. The CPAP rating for 5 amps is probably at 120 volts. That means 600 watts. When you use the proper unit for power, watts, there is no ambiguity about voltage.
If your CPAP is indeed 600 watts, then it is running at about 5 times the power level used for the battery 20 hour capacity measure for 2 batteries. That means you will have a lot less capacity available than specified.
Note that the 'reserve minutes' specification for batteries is at a 25 amp draw. For 12v
batteries, this is 300 watts (or 600 watts for 2 batteries). That is much closer to your CPAP draw (if I have it right) so should be a much better guide for that need.
You can compare the 20 hour and reserve minutes specifications power levels to get a good idea about what changing power levels can do to battery capacity. I have used them to calculate the Peukert coefficient for battery comparisons.
Also note that you should really only use half the specified available energy for optimum battery cost effectiveness.
You know that you need to charge your batteries when their voltage starts to approach 12.0v after resting with no significant charge or discharge for a half hour or more. 12.2v is usually considered the 50% discharged voltage.
I have encountered several folks needing to use CPAP but none have been very satisfied with the typical 2 kWh trailer battery setup as the overnight power source. One, with an FB, tried using the gensets in his truck bed but that set off the CO alarms.
Seems to me, the RV market might be a prime target for CPAP manufacturers but, as far as I know, there aren't any available that are intended for 12v
use and extreme efficiencies.