[Water Warrior 2]

Both of our bikes have weather proof 12 volt sockets so it is easy to pump up a low tire. They are mounted nearer the front of the bike for easy access and fused/wired directly to the battery. A compressor is cheap and easy to replace if it goes south. I carry one under the seat of the Vstrom so it is handy to both bikes.
If your rear tire was that low I would be looking for a nail or some other offending pointy thing. Sounds like you lost at least half the pressure, not a good sign and the temp drop should not affect the pressure that much.
The regular cheap tire pressure guages aren't all that bad. The ones with a square shaped plastic thingy are not bad but the ones that are a flat blade type have not been very accurate in my experience. I check my pressure guages against a very accurate dial type guage a couple times a year. Both bikes and both cages have the same type guage and all seem to be just a tad below the dial guage.