Not sure if my suggestions below will work, but I find it interesting to think "what if..." etc. If it eventually ends up as a success, it is very rewarding to have made what´s "impossible".
A master board with usb connection (for convenience), connected via 30 meter twin wire to a Attiny85 slave with the ultrasonic sensor. If the slave is allowed to sleep between samples, it needs just 5 uA (and 2.5 mA awake @ 1 Mhz). The sensor needs 2 mA (passive) to 15 mA (active).
A super capacitor plus 1 - 2 smaller caps should be able to buffer enough juice to the far end components, depending of how frequent readings will occur. An Attiny running the SoftwareSerial library wakes up from sleep without missing anything in the first incoming byte (that is how I wake up and control a few nodes in my current project - but they are all powered separately).
So you could send commands from the master and recieve replies from the slave. The code in both ends would have to skip all input when it´s own Tx is active.
I am not saying this will work, but it may be worth investigating.
