Since you need reasonable ranges : >10 metres
and very low data rates - might a reading twice a day be adequate?
You could
either combine your signalling and power lines in a single cable, but because of the range use a differential technique for data to reduce noise : RS422 etc. Lots of wiring.
Or make radio coupled devices which sleep most of the time, taking uA or less, and report their measurements via a RF link . A a 'daisy-chain' type polled protocol could work here with large ranges to a central data collection point.
A primary cell could last for many years in this application.
Loads of options
Allan