I probably should have made it clear that the diagnostics event sent 15 minutes before they go offline shows their batteries are over 90% charged, so its not a lack of power in the middle of the night 
Would have been my first thought if I read this.
Anyway, I’ve decided to rework my design and add a watchdog timer that triggers two power switch ICs if there is watchdog kick in the last X minutes. Borrowed the idea and some of the design from this thread: Electron Carrier Board so thanks for that @chipmc!
I will feel much better about deploying units in such remote areas with the addition of the watchdog timer, especially since I’ve been reading threads and it sounds like there are cases where the Cellular modem can get into weird states, and a dance of Cellular.on(), pauses and Cellular.off() seem to be the only way to recover.
The one thing my design is doing that is different than the carrier board is I simply use the watchdog’s RESET line directly to the power chips ON/OFF line, no fancy self-kill from the Electron, and with my firmware, there is no way it would WKUP fast enough to kick the dog in 20ms, so I just ping the DONE line every X minutes, and a full solar/battery disconnect/reconnect will happen if no watchdog kick happens.
Fingers crossed…