We have a beagle who is the same way. We have a long "off season" here and every spring we have to re-train her on her campground manners. It takes some time but it does work. Be consistent, start small (ie standing at the edge of your site) and reward her for good behavior. I've also noticed, that my beagle is much better behaved after a good long hike. Try tiring out your german shepherd before starting training.
As an aside, I really wish the "non dog" people wouldn't chime in on posts like this, unless they have something useful to add (which usually, they DON'T).
I'm so tired of the negativity in these forums.