In 2014 we have stayed at over 25 campgrounds. It has been one of our "traveling/sightseeing" years obviously.
We never had one all year where we could not get a signal from our roof mounted Travler DirectTV dish. We generally don't stay in state or federal campgrounds, but some of the private CG's are wooded. A couple times I looked over the spot they wanted to put us in and asked for a different site because trees could have blocked our signal.
I am not a huge fan of parking under a bunch of trees anyway. You can get all kinds of stuff dropping on your rig plus there is nothing worse than storms and high winds coming through while you are sitting under those trees.