We have been traveling part time in a Class B and B+ for 11 years all around the US, Canada and twice into Mexico. We have experienced petty theft 4 times. The incidents occurred in Tennessee at a KOA, Taos NM, Dubuque IA and Fredericksburg TX. We were able to recover the stolen items in both TN and TX because we saw who took them. Total loss was one fancy door mat and some leveling blocks.
All these cases were in "RV Parks" as opposed to the state or federal campgrounds we prefer and where we had left no other camping gear, so a "scrounger" with questionable morals "assumed" we had left the stuff behind and were theirs for the taking. When we put out chairs, tablecloths, camp stove, etc, we have never had a problem. We always take our power cable and door mat with us but leave the locked surge protector, blocks and TV cable (if used) to show the site is occupied.
I am going to try the sign.