Please see attached preliminary circuit for the desired function. Any discrepancy/ suggestion for the given circuit will be appreciated.
In first post you said that you need to measure average voltage.
You will have guarantee to get average if voltage divider will give output voltage lower than Zener diode voltage.
When Zener starts to conduct you loose average information from your output signal.
As an example consider input signal having 100% fill. If it will be 12V it will certainly have different average voltage than when it is 14V.
But in your circuit for 12V you will get 5.1V then limited to 3.3V and for 14V you will get 6V then limited to 3.3V.
So for two signals of different average voltage you get the same output voltage - you lost average information.