Camper negative and shore power earth ground are tied together!
Where? Your Fridge and if you have a hot water tank that is 120vac and propane.
Lot's of discussion back years ago.. If your interested here is a link.
Or just look at the pictures that I produced along with the notes showing the grounding.
You also will notice that the AC ground and DC- are also tied together at or around the converter.
http://www.picturetrail.com/sfx/album/view/16562094Or if you do not want to go look here is one picture showing the BOND... AC ground to DC-

Not to Hijack this thread but if your interested in the reason I came up with these pictures you can learn from this thread:
http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/19726244/srt/pa/pging/1/page/1.cfmJim