The RF95 driver in RadioHead has no mention of sleep modes for the processor, but in principle it is not hard to implement.
I've done so for the M0, putting it to sleep while waiting for received packets, and the mesh network works, except that exactly every second transmitted packet is received.
There are some weird comments in the RF95 code about skipped interrupts, and the need to clear radio IRQ flags twice, with inconsistent approaches and no clear explanation.
I'm using the ArduinoLowPower library, and suspect problems with it also.
Your ideas would be welcome, so please take a look at what I've done and posted about the problem at https://forums.adafruit.com/viewtopic.php?t=211308