Lots of reading on the subject:
Crime in RV Trailers
KOA Article on crime in campgrounds
Good notes in both articles. They both point out that crime in RV parks is pretty low--you are more likely to encounter something while on the road (which is when Mike got robbed in the post that started this thread).
In our case we simply use common sense: (like mentioned in the articles) Lock the doors, keep the curtains closed, keep all the storage bays locked (751 key notwithstanding), etc. (One of the reasons I got the windshield cover was to keep prying eyes out in the daytime...)
Since we don't fulltime we only carry what we need for that trip (aside from a laptop or two and a phone or two). When we leave the camper our phones are with us so about the only expensive things they could steal are electronics...easily replaceable.