We stayed at one in Hamburg called Knaus camping or similar. It was the second time we'd stayed there - the first time it was great. I was trying with my stilted German to communicate and they were really encouraging. The site was clean and tidy and had great facilities. When we went back last summer there had been a change in management and it wasn't nearly so nice. The people seemed irritated every time I attempted to speak to them, they had closed the small on site shop and generally seemed to not want to talk to us.
I don't know how helpful I've been as that wasn't so much a recommendation as the opposite.
There are lots of really nice campsites on the Bodensee (lake Constance) in the south, just across the border from Switzerland. Can't recommend specific sites as its a few years since I was there, but there's bound to be plenty of info on tripadvisor & similar sites.