How can a 12vdc light cut off my A/C unit?

The A/C unit cuts off when any 12V light is cut on by the light switch....

by "cut off"

do you mean the a/c goes off when your TURN OFF the light

or do you mean you took out the light and killed power on that circuit
which just might be the one feeding the Tstat

Actually the light is not fluorescent....but the typical auto bulb. Ivylog....good thought....have not tried removing the bulb to determine the reaction. I am not an electrical type person but somehow suspect it has to be originating in the power converter....just has to be.,,,I think...LOLOL

I bet it's a florescent light, isn't it. It is the ballast in the light that's sending weird signals to the thermostat.

The AC uses the campers 12v for the Tstat control system. Usually it's when you turn a light on. Does it do it even if there is no light in the socket?