I am used to working with bosh cameras with various heaters and fans.
Your average ptz will produce some btus of heat what can be a problem is a sudden change drop in temperature causing condensation inside the dome bubble.
This can to some degree be dealt with by placing dessicant bags in the camera housing as used in bosch g3 cameras on dome trim ring. The G3 camera has two heatsinks with fins and the convection heat causes circulation throughout the bubble, the outer shield keeps the heat down somewhat as well as a white finish.
The G3 system is far from airtight however there is an optional NEMA gasket for the top of the dome. The perimeter of the bubble does not seal up well.
On g4 camera systems a computer style processor fan. a heater, a theromostat and two plastic ducts are used to blow warm air inside the dome bubble to defog it.
Bosch's ultimate solution is to use nitrogen to pressurize the dome to about 10psi through a valve stem like on a bike tire, this is an expensive option for sure.
One can retrofit some cameras crudely but effectively using various resistive strip heaters and thermostats sometimes scavenged from old pelco prison style camera housings that are used on pan tilt motors.