And problem is that TV needs to have camera GND, not board GND.
This problem is called black level clamping and is caused when the bottom of the sync pulse is a negative voltage and the black level is ground. The way round this is to ac couple the video signal. The use a sync separator and a transistor to short the signal on your side of the capacitor to ground during every sync pulse. This will then allow you to switch both the sync and video signals without affecting the brightness.