12v-4.2v power supply design?
I was thinking of using a 12v to 5v converter and a LT3083 to adjust
  the voltage from 5v to 4.2v, this would also mean i could use a 5v
  battery bank to power the voltage regulator, do you think this will
  work?


Yes, but it isn't the best way. There will be losses with the LDO. Use an adjustable DC-DC converter to go from 12V directly to 4.2V. TI makes lots of parts that are easy to design around.


  Because the camera is quite expensive i want to add over voltage
  protection and a display that shows the estimated battery life, for
  the voltage protection i could use a MCU and a voltage divider to
  measure the voltage on the LT3083 output, if the voltage is correct
  the MCU can turn on a transistor that goes from the voltage regulator
  to the camera,


This will be too slow. By the time the overvoltage is detected, it's likely your camera will be dead. There are several questions about overvoltage protection on here already, do a search.


  ...and i could use another voltage divider to measure the voltage of
  the 12v lead acid to give a estimated battery life.


That is reasonable. I would recommend a high side switch on the voltage divider so you can turn it off when not taking a measurement. Otherwise, it's a constant load on the battery that will slowly drain it.