Relaxation oscilator with only transistor to measure DC voltage
I guess you cannot have a galvanic contact between your 12...100V DC and the rest of your system. The "rest of your system" has only 3,3VDC operating voltage which is used to supply a microcomputer and you try to avoid generating higher voltages from it. I guess the microcomputer has analog input for voltage measurements and it has a way to send the measurement result to some external system that you are not going to present here.

You wrote in a comment one measurement per hour is enough. In that case you can sample the 12...100V DC with a flying capacitor (=both ends moved with a relay) which has a series resistor to keep the current acceptable during the sampling for long relay life. After taking the sample the flying cap is discharged to a pre-discharged bigger capacitor to lower the voltage say to 1...2%. The result is converted to digital.

Capacitors are not precision components but I guess you can choose stable types and calibrate the conversion in software.

12 bit conversion should be enough for the wanted accuracy.

You didn't specify the needed isolation. Miniature relays which can operate at 3V coil voltage do not have especially long gap between the contacts, so it's well possible that relays with long enough isolation gap do not exist.