There are only 6 camera slots available in upper right HUD. Changing first person view is only possible to those cameras (using key 1-6).
A vehicle might have more than 6 cameras mounted, what is pretty useless unless a camera fails during mission.
The Medium Rover has 7 cameras mounted per default.
Per default one of the hazard avoidance cameras is not usable.
If you change the configuration of the rover in a lab, the order of cameras might change, too. I had a mission where I changed the position of the advanced tele focus camera and was not able to use it.