So I finally got a chest freezer and a controller for my keg setup. I have noticed the colder it gets, the lower the pressure gets in the CO2 tank. Is this normal or should I be looking for a leak?
The high pressure guage only reads right at room temp. Just pay attention to the low pressure side. When the high pressure side starts to drop dramatically though, it's time for a refill. Usually you won't notice much change until the bottle is almost empty.
More than likely you don't have a leak. More than likely the drop is due to the lower temperature. A simple way to check is, if your beer is fully carbonated, turn off the cylinder before you go to bed (note pressure), take a look at it the next time your pulling a pint. If the pressure is the same your good.
One thing to notice from this chart (very cool, btw), it doesn't ever drop below 500 psi until you have about 10% remaining. By this point, its basically empty. The moral of the story is that the "high side" gauge is basically useless. It will stay a mostly constant pressure (variable by temp) until its almost gone and then drop rapidly. This means the gauge is only worthwhile for verifying suspicions that its empty.