I have a Wahoo Trackr Radar, which I have yet to try out on a tour (that will be in July), but one feature I like is that I can set it to turn off if it isn't detecting a vehicle behind me. Once it detects a vehicle, it goes into flash mode until all is clear. Based on my rather limited local riding with it, up to four hours at a time, I estimate with a safety margin that I will get at least two days of touring out of it. My touring days, however, are shorter than yours, generally around a maximum of eight hours per day.

I also ride with a Mirracycle mirror, as I like to know not only that I have a vehicle behind me (radar), but also what it is doing (mirror).

It does not help that Perth is a very windy city so relying on hearing is a high-risk strategy at the best of times. I have had a hotted-up V8 come past me without hearing it due to wind noise. Further, I have had a dude get upset at me as I caused him to wait at an intersection for me to pass through, so he swung after me and tried to run me over. My mirror saved me that day; my hearing did not help at all due to the traffic. Thankfully, I timed the jump over the kerb to safety well.

Edit: For what it is worth, I am finding my Trackr Radar does not detect cyclists behind me. We do not have mountain lions so cannot comment on that aspect